876 lines
26 KiB
C
876 lines
26 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* KFENCE guarded object allocator and fault handling.
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*
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* Copyright (C) 2020, Google LLC.
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*/
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#define pr_fmt(fmt) "kfence: " fmt
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#include <linux/atomic.h>
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#include <linux/bug.h>
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#include <linux/debugfs.h>
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#include <linux/irq_work.h>
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#include <linux/kcsan-checks.h>
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#include <linux/kfence.h>
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#include <linux/kmemleak.h>
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#include <linux/list.h>
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#include <linux/lockdep.h>
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#include <linux/memblock.h>
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#include <linux/moduleparam.h>
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#include <linux/random.h>
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#include <linux/rcupdate.h>
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#include <linux/sched/sysctl.h>
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#include <linux/seq_file.h>
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#include <linux/slab.h>
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#include <linux/spinlock.h>
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#include <linux/string.h>
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#include <asm/kfence.h>
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#include "kfence.h"
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/* Disables KFENCE on the first warning assuming an irrecoverable error. */
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#define KFENCE_WARN_ON(cond) \
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({ \
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const bool __cond = WARN_ON(cond); \
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if (unlikely(__cond)) \
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WRITE_ONCE(kfence_enabled, false); \
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__cond; \
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})
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/* === Data ================================================================= */
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static bool kfence_enabled __read_mostly;
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static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
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#ifdef MODULE_PARAM_PREFIX
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#undef MODULE_PARAM_PREFIX
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#endif
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#define MODULE_PARAM_PREFIX "kfence."
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static int param_set_sample_interval(const char *val, const struct kernel_param *kp)
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{
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unsigned long num;
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int ret = kstrtoul(val, 0, &num);
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if (ret < 0)
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return ret;
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if (!num) /* Using 0 to indicate KFENCE is disabled. */
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WRITE_ONCE(kfence_enabled, false);
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else if (!READ_ONCE(kfence_enabled) && system_state != SYSTEM_BOOTING)
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return -EINVAL; /* Cannot (re-)enable KFENCE on-the-fly. */
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*((unsigned long *)kp->arg) = num;
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return 0;
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}
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static int param_get_sample_interval(char *buffer, const struct kernel_param *kp)
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{
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if (!READ_ONCE(kfence_enabled))
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return sprintf(buffer, "0\n");
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return param_get_ulong(buffer, kp);
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}
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static const struct kernel_param_ops sample_interval_param_ops = {
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.set = param_set_sample_interval,
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.get = param_get_sample_interval,
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};
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module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_interval, 0600);
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/* The pool of pages used for guard pages and objects. */
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char *__kfence_pool __ro_after_init;
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EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
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/*
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* Per-object metadata, with one-to-one mapping of object metadata to
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* backing pages (in __kfence_pool).
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*/
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static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
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struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
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/* Freelist with available objects. */
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static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
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static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
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#ifdef CONFIG_KFENCE_STATIC_KEYS
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/* The static key to set up a KFENCE allocation. */
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DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
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#endif
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/* Gates the allocation, ensuring only one succeeds in a given period. */
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atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
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/* Statistics counters for debugfs. */
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enum kfence_counter_id {
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KFENCE_COUNTER_ALLOCATED,
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KFENCE_COUNTER_ALLOCS,
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KFENCE_COUNTER_FREES,
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KFENCE_COUNTER_ZOMBIES,
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KFENCE_COUNTER_BUGS,
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KFENCE_COUNTER_COUNT,
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};
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static atomic_long_t counters[KFENCE_COUNTER_COUNT];
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static const char *const counter_names[] = {
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[KFENCE_COUNTER_ALLOCATED] = "currently allocated",
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[KFENCE_COUNTER_ALLOCS] = "total allocations",
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[KFENCE_COUNTER_FREES] = "total frees",
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[KFENCE_COUNTER_ZOMBIES] = "zombie allocations",
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[KFENCE_COUNTER_BUGS] = "total bugs",
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};
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static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
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/* === Internals ============================================================ */
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static bool kfence_protect(unsigned long addr)
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{
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return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
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}
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static bool kfence_unprotect(unsigned long addr)
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{
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return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
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}
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static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
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{
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long index;
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/* The checks do not affect performance; only called from slow-paths. */
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if (!is_kfence_address((void *)addr))
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return NULL;
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/*
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* May be an invalid index if called with an address at the edge of
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* __kfence_pool, in which case we would report an "invalid access"
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* error.
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*/
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index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
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if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
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return NULL;
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return &kfence_metadata[index];
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}
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static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
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{
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unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
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unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
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/* The checks do not affect performance; only called from slow-paths. */
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/* Only call with a pointer into kfence_metadata. */
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if (KFENCE_WARN_ON(meta < kfence_metadata ||
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meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
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return 0;
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/*
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* This metadata object only ever maps to 1 page; verify that the stored
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* address is in the expected range.
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*/
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if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
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return 0;
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return pageaddr;
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}
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/*
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* Update the object's metadata state, including updating the alloc/free stacks
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* depending on the state transition.
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*/
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static noinline void metadata_update_state(struct kfence_metadata *meta,
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enum kfence_object_state next)
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{
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struct kfence_track *track =
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next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
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lockdep_assert_held(&meta->lock);
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/*
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* Skip over 1 (this) functions; noinline ensures we do not accidentally
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* skip over the caller by never inlining.
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*/
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track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
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track->pid = task_pid_nr(current);
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/*
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* Pairs with READ_ONCE() in
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* kfence_shutdown_cache(),
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* kfence_handle_page_fault().
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*/
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WRITE_ONCE(meta->state, next);
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}
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/* Write canary byte to @addr. */
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static inline bool set_canary_byte(u8 *addr)
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{
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*addr = KFENCE_CANARY_PATTERN(addr);
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return true;
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}
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/* Check canary byte at @addr. */
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static inline bool check_canary_byte(u8 *addr)
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{
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if (likely(*addr == KFENCE_CANARY_PATTERN(addr)))
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return true;
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atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
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kfence_report_error((unsigned long)addr, false, NULL, addr_to_metadata((unsigned long)addr),
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KFENCE_ERROR_CORRUPTION);
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return false;
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}
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/* __always_inline this to ensure we won't do an indirect call to fn. */
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static __always_inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
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{
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const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
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unsigned long addr;
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lockdep_assert_held(&meta->lock);
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/*
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* We'll iterate over each canary byte per-side until fn() returns
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* false. However, we'll still iterate over the canary bytes to the
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* right of the object even if there was an error in the canary bytes to
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* the left of the object. Specifically, if check_canary_byte()
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* generates an error, showing both sides might give more clues as to
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* what the error is about when displaying which bytes were corrupted.
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*/
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/* Apply to left of object. */
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for (addr = pageaddr; addr < meta->addr; addr++) {
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if (!fn((u8 *)addr))
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break;
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}
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/* Apply to right of object. */
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for (addr = meta->addr + meta->size; addr < pageaddr + PAGE_SIZE; addr++) {
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if (!fn((u8 *)addr))
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break;
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}
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}
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static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
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{
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struct kfence_metadata *meta = NULL;
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unsigned long flags;
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struct page *page;
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void *addr;
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/* Try to obtain a free object. */
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raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
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if (!list_empty(&kfence_freelist)) {
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meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
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list_del_init(&meta->list);
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}
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raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
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if (!meta)
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return NULL;
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if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
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/*
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* This is extremely unlikely -- we are reporting on a
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* use-after-free, which locked meta->lock, and the reporting
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* code via printk calls kmalloc() which ends up in
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* kfence_alloc() and tries to grab the same object that we're
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* reporting on. While it has never been observed, lockdep does
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* report that there is a possibility of deadlock. Fix it by
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* using trylock and bailing out gracefully.
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*/
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raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
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/* Put the object back on the freelist. */
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list_add_tail(&meta->list, &kfence_freelist);
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raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
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return NULL;
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}
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meta->addr = metadata_to_pageaddr(meta);
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/* Unprotect if we're reusing this page. */
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if (meta->state == KFENCE_OBJECT_FREED)
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kfence_unprotect(meta->addr);
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/*
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* Note: for allocations made before RNG initialization, will always
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* return zero. We still benefit from enabling KFENCE as early as
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* possible, even when the RNG is not yet available, as this will allow
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* KFENCE to detect bugs due to earlier allocations. The only downside
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* is that the out-of-bounds accesses detected are deterministic for
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* such allocations.
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*/
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if (prandom_u32_max(2)) {
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/* Allocate on the "right" side, re-calculate address. */
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meta->addr += PAGE_SIZE - size;
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meta->addr = ALIGN_DOWN(meta->addr, cache->align);
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}
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addr = (void *)meta->addr;
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/* Update remaining metadata. */
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metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
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/* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
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WRITE_ONCE(meta->cache, cache);
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meta->size = size;
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for_each_canary(meta, set_canary_byte);
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/* Set required struct page fields. */
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page = virt_to_page(meta->addr);
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page->slab_cache = cache;
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if (IS_ENABLED(CONFIG_SLUB))
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page->objects = 1;
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if (IS_ENABLED(CONFIG_SLAB))
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page->s_mem = addr;
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raw_spin_unlock_irqrestore(&meta->lock, flags);
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/* Memory initialization. */
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/*
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* We check slab_want_init_on_alloc() ourselves, rather than letting
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* SL*B do the initialization, as otherwise we might overwrite KFENCE's
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* redzone.
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*/
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if (unlikely(slab_want_init_on_alloc(gfp, cache)))
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memzero_explicit(addr, size);
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if (cache->ctor)
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cache->ctor(addr);
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if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
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kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
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atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
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atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
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return addr;
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}
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static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
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{
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struct kcsan_scoped_access assert_page_exclusive;
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unsigned long flags;
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raw_spin_lock_irqsave(&meta->lock, flags);
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if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
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/* Invalid or double-free, bail out. */
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atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
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kfence_report_error((unsigned long)addr, false, NULL, meta,
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KFENCE_ERROR_INVALID_FREE);
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raw_spin_unlock_irqrestore(&meta->lock, flags);
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return;
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}
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/* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
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kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
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KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
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&assert_page_exclusive);
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if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
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kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
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/* Restore page protection if there was an OOB access. */
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if (meta->unprotected_page) {
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memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
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kfence_protect(meta->unprotected_page);
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meta->unprotected_page = 0;
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}
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/* Check canary bytes for memory corruption. */
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for_each_canary(meta, check_canary_byte);
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/*
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* Clear memory if init-on-free is set. While we protect the page, the
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* data is still there, and after a use-after-free is detected, we
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* unprotect the page, so the data is still accessible.
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*/
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if (!zombie && unlikely(slab_want_init_on_free(meta->cache)))
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memzero_explicit(addr, meta->size);
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/* Mark the object as freed. */
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metadata_update_state(meta, KFENCE_OBJECT_FREED);
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raw_spin_unlock_irqrestore(&meta->lock, flags);
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/* Protect to detect use-after-frees. */
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kfence_protect((unsigned long)addr);
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kcsan_end_scoped_access(&assert_page_exclusive);
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if (!zombie) {
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/* Add it to the tail of the freelist for reuse. */
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raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
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KFENCE_WARN_ON(!list_empty(&meta->list));
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list_add_tail(&meta->list, &kfence_freelist);
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raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
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atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
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atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
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} else {
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/* See kfence_shutdown_cache(). */
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atomic_long_inc(&counters[KFENCE_COUNTER_ZOMBIES]);
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}
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}
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static void rcu_guarded_free(struct rcu_head *h)
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{
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struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
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kfence_guarded_free((void *)meta->addr, meta, false);
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}
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static bool __init kfence_init_pool(void)
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{
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unsigned long addr = (unsigned long)__kfence_pool;
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struct page *pages;
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int i;
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if (!__kfence_pool)
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return false;
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if (!arch_kfence_init_pool())
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goto err;
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pages = virt_to_page(addr);
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/*
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* Set up object pages: they must have PG_slab set, to avoid freeing
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* these as real pages.
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*
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* We also want to avoid inserting kfence_free() in the kfree()
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* fast-path in SLUB, and therefore need to ensure kfree() correctly
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* enters __slab_free() slow-path.
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*/
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for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
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if (!i || (i % 2))
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continue;
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/* Verify we do not have a compound head page. */
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if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
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goto err;
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__SetPageSlab(&pages[i]);
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}
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/*
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* Protect the first 2 pages. The first page is mostly unnecessary, and
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* merely serves as an extended guard page. However, adding one
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* additional page in the beginning gives us an even number of pages,
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* which simplifies the mapping of address to metadata index.
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*/
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for (i = 0; i < 2; i++) {
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if (unlikely(!kfence_protect(addr)))
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goto err;
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addr += PAGE_SIZE;
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}
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for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
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struct kfence_metadata *meta = &kfence_metadata[i];
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/* Initialize metadata. */
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INIT_LIST_HEAD(&meta->list);
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raw_spin_lock_init(&meta->lock);
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meta->state = KFENCE_OBJECT_UNUSED;
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meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
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list_add_tail(&meta->list, &kfence_freelist);
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/* Protect the right redzone. */
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if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
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goto err;
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addr += 2 * PAGE_SIZE;
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}
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/*
|
|
* The pool is live and will never be deallocated from this point on.
|
|
* Remove the pool object from the kmemleak object tree, as it would
|
|
* otherwise overlap with allocations returned by kfence_alloc(), which
|
|
* are registered with kmemleak through the slab post-alloc hook.
|
|
*/
|
|
kmemleak_free(__kfence_pool);
|
|
|
|
return true;
|
|
|
|
err:
|
|
/*
|
|
* Only release unprotected pages, and do not try to go back and change
|
|
* page attributes due to risk of failing to do so as well. If changing
|
|
* page attributes for some pages fails, it is very likely that it also
|
|
* fails for the first page, and therefore expect addr==__kfence_pool in
|
|
* most failure cases.
|
|
*/
|
|
memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
|
|
__kfence_pool = NULL;
|
|
return false;
|
|
}
|
|
|
|
/* === DebugFS Interface ==================================================== */
|
|
|
|
static int stats_show(struct seq_file *seq, void *v)
|
|
{
|
|
int i;
|
|
|
|
seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
|
|
for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
|
|
seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
|
|
|
|
return 0;
|
|
}
|
|
DEFINE_SHOW_ATTRIBUTE(stats);
|
|
|
|
/*
|
|
* debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
|
|
* start_object() and next_object() return the object index + 1, because NULL is used
|
|
* to stop iteration.
|
|
*/
|
|
static void *start_object(struct seq_file *seq, loff_t *pos)
|
|
{
|
|
if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
|
|
return (void *)((long)*pos + 1);
|
|
return NULL;
|
|
}
|
|
|
|
static void stop_object(struct seq_file *seq, void *v)
|
|
{
|
|
}
|
|
|
|
static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
|
|
{
|
|
++*pos;
|
|
if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
|
|
return (void *)((long)*pos + 1);
|
|
return NULL;
|
|
}
|
|
|
|
static int show_object(struct seq_file *seq, void *v)
|
|
{
|
|
struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
|
|
unsigned long flags;
|
|
|
|
raw_spin_lock_irqsave(&meta->lock, flags);
|
|
kfence_print_object(seq, meta);
|
|
raw_spin_unlock_irqrestore(&meta->lock, flags);
|
|
seq_puts(seq, "---------------------------------\n");
|
|
|
|
return 0;
|
|
}
|
|
|
|
static const struct seq_operations object_seqops = {
|
|
.start = start_object,
|
|
.next = next_object,
|
|
.stop = stop_object,
|
|
.show = show_object,
|
|
};
|
|
|
|
static int open_objects(struct inode *inode, struct file *file)
|
|
{
|
|
return seq_open(file, &object_seqops);
|
|
}
|
|
|
|
static const struct file_operations objects_fops = {
|
|
.open = open_objects,
|
|
.read = seq_read,
|
|
.llseek = seq_lseek,
|
|
};
|
|
|
|
static int __init kfence_debugfs_init(void)
|
|
{
|
|
struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
|
|
|
|
debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
|
|
debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
|
|
return 0;
|
|
}
|
|
|
|
late_initcall(kfence_debugfs_init);
|
|
|
|
/* === Allocation Gate Timer ================================================ */
|
|
|
|
#ifdef CONFIG_KFENCE_STATIC_KEYS
|
|
/* Wait queue to wake up allocation-gate timer task. */
|
|
static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
|
|
|
|
static void wake_up_kfence_timer(struct irq_work *work)
|
|
{
|
|
wake_up(&allocation_wait);
|
|
}
|
|
static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
|
|
#endif
|
|
|
|
/*
|
|
* Set up delayed work, which will enable and disable the static key. We need to
|
|
* use a work queue (rather than a simple timer), since enabling and disabling a
|
|
* static key cannot be done from an interrupt.
|
|
*
|
|
* Note: Toggling a static branch currently causes IPIs, and here we'll end up
|
|
* with a total of 2 IPIs to all CPUs. If this ends up a problem in future (with
|
|
* more aggressive sampling intervals), we could get away with a variant that
|
|
* avoids IPIs, at the cost of not immediately capturing allocations if the
|
|
* instructions remain cached.
|
|
*/
|
|
static struct delayed_work kfence_timer;
|
|
static void toggle_allocation_gate(struct work_struct *work)
|
|
{
|
|
if (!READ_ONCE(kfence_enabled))
|
|
return;
|
|
|
|
atomic_set(&kfence_allocation_gate, 0);
|
|
#ifdef CONFIG_KFENCE_STATIC_KEYS
|
|
/* Enable static key, and await allocation to happen. */
|
|
static_branch_enable(&kfence_allocation_key);
|
|
|
|
if (sysctl_hung_task_timeout_secs) {
|
|
/*
|
|
* During low activity with no allocations we might wait a
|
|
* while; let's avoid the hung task warning.
|
|
*/
|
|
wait_event_idle_timeout(allocation_wait, atomic_read(&kfence_allocation_gate),
|
|
sysctl_hung_task_timeout_secs * HZ / 2);
|
|
} else {
|
|
wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate));
|
|
}
|
|
|
|
/* Disable static key and reset timer. */
|
|
static_branch_disable(&kfence_allocation_key);
|
|
#endif
|
|
queue_delayed_work(system_power_efficient_wq, &kfence_timer,
|
|
msecs_to_jiffies(kfence_sample_interval));
|
|
}
|
|
static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
|
|
|
|
/* === Public interface ===================================================== */
|
|
|
|
void __init kfence_alloc_pool(void)
|
|
{
|
|
if (!kfence_sample_interval)
|
|
return;
|
|
|
|
__kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
|
|
|
|
if (!__kfence_pool)
|
|
pr_err("failed to allocate pool\n");
|
|
}
|
|
|
|
void __init kfence_init(void)
|
|
{
|
|
/* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
|
|
if (!kfence_sample_interval)
|
|
return;
|
|
|
|
if (!kfence_init_pool()) {
|
|
pr_err("%s failed\n", __func__);
|
|
return;
|
|
}
|
|
|
|
WRITE_ONCE(kfence_enabled, true);
|
|
queue_delayed_work(system_power_efficient_wq, &kfence_timer, 0);
|
|
pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
|
|
CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
|
|
(void *)(__kfence_pool + KFENCE_POOL_SIZE));
|
|
}
|
|
|
|
void kfence_shutdown_cache(struct kmem_cache *s)
|
|
{
|
|
unsigned long flags;
|
|
struct kfence_metadata *meta;
|
|
int i;
|
|
|
|
for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
|
|
bool in_use;
|
|
|
|
meta = &kfence_metadata[i];
|
|
|
|
/*
|
|
* If we observe some inconsistent cache and state pair where we
|
|
* should have returned false here, cache destruction is racing
|
|
* with either kmem_cache_alloc() or kmem_cache_free(). Taking
|
|
* the lock will not help, as different critical section
|
|
* serialization will have the same outcome.
|
|
*/
|
|
if (READ_ONCE(meta->cache) != s ||
|
|
READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
|
|
continue;
|
|
|
|
raw_spin_lock_irqsave(&meta->lock, flags);
|
|
in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
|
|
raw_spin_unlock_irqrestore(&meta->lock, flags);
|
|
|
|
if (in_use) {
|
|
/*
|
|
* This cache still has allocations, and we should not
|
|
* release them back into the freelist so they can still
|
|
* safely be used and retain the kernel's default
|
|
* behaviour of keeping the allocations alive (leak the
|
|
* cache); however, they effectively become "zombie
|
|
* allocations" as the KFENCE objects are the only ones
|
|
* still in use and the owning cache is being destroyed.
|
|
*
|
|
* We mark them freed, so that any subsequent use shows
|
|
* more useful error messages that will include stack
|
|
* traces of the user of the object, the original
|
|
* allocation, and caller to shutdown_cache().
|
|
*/
|
|
kfence_guarded_free((void *)meta->addr, meta, /*zombie=*/true);
|
|
}
|
|
}
|
|
|
|
for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
|
|
meta = &kfence_metadata[i];
|
|
|
|
/* See above. */
|
|
if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
|
|
continue;
|
|
|
|
raw_spin_lock_irqsave(&meta->lock, flags);
|
|
if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
|
|
meta->cache = NULL;
|
|
raw_spin_unlock_irqrestore(&meta->lock, flags);
|
|
}
|
|
}
|
|
|
|
void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
|
|
{
|
|
/*
|
|
* allocation_gate only needs to become non-zero, so it doesn't make
|
|
* sense to continue writing to it and pay the associated contention
|
|
* cost, in case we have a large number of concurrent allocations.
|
|
*/
|
|
if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
|
|
return NULL;
|
|
#ifdef CONFIG_KFENCE_STATIC_KEYS
|
|
/*
|
|
* waitqueue_active() is fully ordered after the update of
|
|
* kfence_allocation_gate per atomic_inc_return().
|
|
*/
|
|
if (waitqueue_active(&allocation_wait)) {
|
|
/*
|
|
* Calling wake_up() here may deadlock when allocations happen
|
|
* from within timer code. Use an irq_work to defer it.
|
|
*/
|
|
irq_work_queue(&wake_up_kfence_timer_work);
|
|
}
|
|
#endif
|
|
|
|
if (!READ_ONCE(kfence_enabled))
|
|
return NULL;
|
|
|
|
if (size > PAGE_SIZE)
|
|
return NULL;
|
|
|
|
return kfence_guarded_alloc(s, size, flags);
|
|
}
|
|
|
|
size_t kfence_ksize(const void *addr)
|
|
{
|
|
const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
|
|
|
|
/*
|
|
* Read locklessly -- if there is a race with __kfence_alloc(), this is
|
|
* either a use-after-free or invalid access.
|
|
*/
|
|
return meta ? meta->size : 0;
|
|
}
|
|
|
|
void *kfence_object_start(const void *addr)
|
|
{
|
|
const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
|
|
|
|
/*
|
|
* Read locklessly -- if there is a race with __kfence_alloc(), this is
|
|
* either a use-after-free or invalid access.
|
|
*/
|
|
return meta ? (void *)meta->addr : NULL;
|
|
}
|
|
|
|
void __kfence_free(void *addr)
|
|
{
|
|
struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
|
|
|
|
/*
|
|
* If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
|
|
* the object, as the object page may be recycled for other-typed
|
|
* objects once it has been freed. meta->cache may be NULL if the cache
|
|
* was destroyed.
|
|
*/
|
|
if (unlikely(meta->cache && (meta->cache->flags & SLAB_TYPESAFE_BY_RCU)))
|
|
call_rcu(&meta->rcu_head, rcu_guarded_free);
|
|
else
|
|
kfence_guarded_free(addr, meta, false);
|
|
}
|
|
|
|
bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs *regs)
|
|
{
|
|
const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
|
|
struct kfence_metadata *to_report = NULL;
|
|
enum kfence_error_type error_type;
|
|
unsigned long flags;
|
|
|
|
if (!is_kfence_address((void *)addr))
|
|
return false;
|
|
|
|
if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
|
|
return kfence_unprotect(addr); /* ... unprotect and proceed. */
|
|
|
|
atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
|
|
|
|
if (page_index % 2) {
|
|
/* This is a redzone, report a buffer overflow. */
|
|
struct kfence_metadata *meta;
|
|
int distance = 0;
|
|
|
|
meta = addr_to_metadata(addr - PAGE_SIZE);
|
|
if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
|
|
to_report = meta;
|
|
/* Data race ok; distance calculation approximate. */
|
|
distance = addr - data_race(meta->addr + meta->size);
|
|
}
|
|
|
|
meta = addr_to_metadata(addr + PAGE_SIZE);
|
|
if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
|
|
/* Data race ok; distance calculation approximate. */
|
|
if (!to_report || distance > data_race(meta->addr) - addr)
|
|
to_report = meta;
|
|
}
|
|
|
|
if (!to_report)
|
|
goto out;
|
|
|
|
raw_spin_lock_irqsave(&to_report->lock, flags);
|
|
to_report->unprotected_page = addr;
|
|
error_type = KFENCE_ERROR_OOB;
|
|
|
|
/*
|
|
* If the object was freed before we took the look we can still
|
|
* report this as an OOB -- the report will simply show the
|
|
* stacktrace of the free as well.
|
|
*/
|
|
} else {
|
|
to_report = addr_to_metadata(addr);
|
|
if (!to_report)
|
|
goto out;
|
|
|
|
raw_spin_lock_irqsave(&to_report->lock, flags);
|
|
error_type = KFENCE_ERROR_UAF;
|
|
/*
|
|
* We may race with __kfence_alloc(), and it is possible that a
|
|
* freed object may be reallocated. We simply report this as a
|
|
* use-after-free, with the stack trace showing the place where
|
|
* the object was re-allocated.
|
|
*/
|
|
}
|
|
|
|
out:
|
|
if (to_report) {
|
|
kfence_report_error(addr, is_write, regs, to_report, error_type);
|
|
raw_spin_unlock_irqrestore(&to_report->lock, flags);
|
|
} else {
|
|
/* This may be a UAF or OOB access, but we can't be sure. */
|
|
kfence_report_error(addr, is_write, regs, NULL, KFENCE_ERROR_INVALID);
|
|
}
|
|
|
|
return kfence_unprotect(addr); /* Unprotect and let access proceed. */
|
|
}
|