208 lines
6.2 KiB
C
208 lines
6.2 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/* ----------------------------------------------------------------------- *
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*
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* Copyright 2014 Intel Corporation; author: H. Peter Anvin
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*
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* ----------------------------------------------------------------------- */
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/*
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* The IRET instruction, when returning to a 16-bit segment, only
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* restores the bottom 16 bits of the user space stack pointer. This
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* causes some 16-bit software to break, but it also leaks kernel state
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* to user space.
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*
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* This works around this by creating percpu "ministacks", each of which
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* is mapped 2^16 times 64K apart. When we detect that the return SS is
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* on the LDT, we copy the IRET frame to the ministack and use the
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* relevant alias to return to userspace. The ministacks are mapped
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* readonly, so if the IRET fault we promote #GP to #DF which is an IST
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* vector and thus has its own stack; we then do the fixup in the #DF
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* handler.
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*
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* This file sets up the ministacks and the related page tables. The
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* actual ministack invocation is in entry_64.S.
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*/
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#include <linux/init.h>
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#include <linux/init_task.h>
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#include <linux/kernel.h>
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#include <linux/percpu.h>
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#include <linux/gfp.h>
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#include <linux/random.h>
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#include <asm/pgtable.h>
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#include <asm/pgalloc.h>
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#include <asm/setup.h>
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#include <asm/espfix.h>
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/*
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* Note: we only need 6*8 = 48 bytes for the espfix stack, but round
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* it up to a cache line to avoid unnecessary sharing.
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*/
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#define ESPFIX_STACK_SIZE (8*8UL)
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#define ESPFIX_STACKS_PER_PAGE (PAGE_SIZE/ESPFIX_STACK_SIZE)
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/* There is address space for how many espfix pages? */
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#define ESPFIX_PAGE_SPACE (1UL << (P4D_SHIFT-PAGE_SHIFT-16))
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#define ESPFIX_MAX_CPUS (ESPFIX_STACKS_PER_PAGE * ESPFIX_PAGE_SPACE)
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#if CONFIG_NR_CPUS > ESPFIX_MAX_CPUS
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# error "Need more virtual address space for the ESPFIX hack"
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#endif
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#define PGALLOC_GFP (GFP_KERNEL | __GFP_ZERO)
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/* This contains the *bottom* address of the espfix stack */
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DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack);
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DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr);
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/* Initialization mutex - should this be a spinlock? */
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static DEFINE_MUTEX(espfix_init_mutex);
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/* Page allocation bitmap - each page serves ESPFIX_STACKS_PER_PAGE CPUs */
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#define ESPFIX_MAX_PAGES DIV_ROUND_UP(CONFIG_NR_CPUS, ESPFIX_STACKS_PER_PAGE)
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static void *espfix_pages[ESPFIX_MAX_PAGES];
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static __page_aligned_bss pud_t espfix_pud_page[PTRS_PER_PUD]
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__aligned(PAGE_SIZE);
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static unsigned int page_random, slot_random;
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/*
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* This returns the bottom address of the espfix stack for a specific CPU.
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* The math allows for a non-power-of-two ESPFIX_STACK_SIZE, in which case
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* we have to account for some amount of padding at the end of each page.
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*/
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static inline unsigned long espfix_base_addr(unsigned int cpu)
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{
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unsigned long page, slot;
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unsigned long addr;
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page = (cpu / ESPFIX_STACKS_PER_PAGE) ^ page_random;
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slot = (cpu + slot_random) % ESPFIX_STACKS_PER_PAGE;
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addr = (page << PAGE_SHIFT) + (slot * ESPFIX_STACK_SIZE);
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addr = (addr & 0xffffUL) | ((addr & ~0xffffUL) << 16);
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addr += ESPFIX_BASE_ADDR;
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return addr;
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}
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#define PTE_STRIDE (65536/PAGE_SIZE)
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#define ESPFIX_PTE_CLONES (PTRS_PER_PTE/PTE_STRIDE)
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#define ESPFIX_PMD_CLONES PTRS_PER_PMD
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#define ESPFIX_PUD_CLONES (65536/(ESPFIX_PTE_CLONES*ESPFIX_PMD_CLONES))
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#define PGTABLE_PROT ((_KERNPG_TABLE & ~_PAGE_RW) | _PAGE_NX)
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static void init_espfix_random(void)
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{
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unsigned long rand;
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/*
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* This is run before the entropy pools are initialized,
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* but this is hopefully better than nothing.
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*/
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if (!arch_get_random_long(&rand)) {
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/* The constant is an arbitrary large prime */
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rand = rdtsc();
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rand *= 0xc345c6b72fd16123UL;
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}
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slot_random = rand % ESPFIX_STACKS_PER_PAGE;
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page_random = (rand / ESPFIX_STACKS_PER_PAGE)
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& (ESPFIX_PAGE_SPACE - 1);
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}
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void __init init_espfix_bsp(void)
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{
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pgd_t *pgd;
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p4d_t *p4d;
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/* Install the espfix pud into the kernel page directory */
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pgd = &init_top_pgt[pgd_index(ESPFIX_BASE_ADDR)];
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p4d = p4d_alloc(&init_mm, pgd, ESPFIX_BASE_ADDR);
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p4d_populate(&init_mm, p4d, espfix_pud_page);
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/* Randomize the locations */
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init_espfix_random();
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/* The rest is the same as for any other processor */
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init_espfix_ap(0);
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}
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void init_espfix_ap(int cpu)
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{
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unsigned int page;
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unsigned long addr;
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pud_t pud, *pud_p;
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pmd_t pmd, *pmd_p;
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pte_t pte, *pte_p;
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int n, node;
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void *stack_page;
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pteval_t ptemask;
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/* We only have to do this once... */
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if (likely(per_cpu(espfix_stack, cpu)))
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return; /* Already initialized */
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addr = espfix_base_addr(cpu);
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page = cpu/ESPFIX_STACKS_PER_PAGE;
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/* Did another CPU already set this up? */
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stack_page = READ_ONCE(espfix_pages[page]);
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if (likely(stack_page))
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goto done;
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mutex_lock(&espfix_init_mutex);
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/* Did we race on the lock? */
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stack_page = READ_ONCE(espfix_pages[page]);
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if (stack_page)
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goto unlock_done;
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node = cpu_to_node(cpu);
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ptemask = __supported_pte_mask;
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pud_p = &espfix_pud_page[pud_index(addr)];
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pud = *pud_p;
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if (!pud_present(pud)) {
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struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
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pmd_p = (pmd_t *)page_address(page);
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pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
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paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
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for (n = 0; n < ESPFIX_PUD_CLONES; n++)
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set_pud(&pud_p[n], pud);
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}
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pmd_p = pmd_offset(&pud, addr);
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pmd = *pmd_p;
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if (!pmd_present(pmd)) {
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struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
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pte_p = (pte_t *)page_address(page);
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pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
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paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
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for (n = 0; n < ESPFIX_PMD_CLONES; n++)
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set_pmd(&pmd_p[n], pmd);
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}
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pte_p = pte_offset_kernel(&pmd, addr);
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stack_page = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
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/*
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* __PAGE_KERNEL_* includes _PAGE_GLOBAL, which we want since
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* this is mapped to userspace.
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*/
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pte = __pte(__pa(stack_page) | ((__PAGE_KERNEL_RO | _PAGE_ENC) & ptemask));
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for (n = 0; n < ESPFIX_PTE_CLONES; n++)
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set_pte(&pte_p[n*PTE_STRIDE], pte);
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/* Job is done for this CPU and any CPU which shares this page */
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WRITE_ONCE(espfix_pages[page], stack_page);
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unlock_done:
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mutex_unlock(&espfix_init_mutex);
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done:
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per_cpu(espfix_stack, cpu) = addr;
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per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
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+ (addr & ~PAGE_MASK);
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}
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