llvm-project/compiler-rt/lib/cfi/cfi.cpp

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//===-------- cfi.cpp -----------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the runtime support for the cross-DSO CFI.
//
//===----------------------------------------------------------------------===//
#include <assert.h>
#include <elf.h>
#include "sanitizer_common/sanitizer_common.h"
#if SANITIZER_FREEBSD
#include <sys/link_elf.h>
#endif
#include <link.h>
#include <string.h>
#include <stdlib.h>
#include <sys/mman.h>
#if SANITIZER_LINUX
typedef ElfW(Phdr) Elf_Phdr;
typedef ElfW(Ehdr) Elf_Ehdr;
typedef ElfW(Addr) Elf_Addr;
typedef ElfW(Sym) Elf_Sym;
typedef ElfW(Dyn) Elf_Dyn;
#elif SANITIZER_FREEBSD
#if SANITIZER_WORDSIZE == 64
#define ElfW64_Dyn Elf_Dyn
#define ElfW64_Sym Elf_Sym
#else
#define ElfW32_Dyn Elf_Dyn
#define ElfW32_Sym Elf_Sym
#endif
#endif
#include "interception/interception.h"
#include "sanitizer_common/sanitizer_flag_parser.h"
#include "ubsan/ubsan_init.h"
#include "ubsan/ubsan_flags.h"
#ifdef CFI_ENABLE_DIAG
#include "ubsan/ubsan_handlers.h"
#endif
using namespace __sanitizer;
namespace __cfi {
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#define kCfiShadowLimitsStorageSize 4096 // 1 page
// Lets hope that the data segment is mapped with 4K pages.
// The pointer to the cfi shadow region is stored at the start of this page.
// The rest of the page is unused and re-mapped read-only.
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static union {
char space[kCfiShadowLimitsStorageSize];
struct {
uptr start;
uptr size;
} limits;
} cfi_shadow_limits_storage
__attribute__((aligned(kCfiShadowLimitsStorageSize)));
static constexpr uptr kShadowGranularity = 12;
static constexpr uptr kShadowAlign = 1UL << kShadowGranularity; // 4096
static constexpr uint16_t kInvalidShadow = 0;
static constexpr uint16_t kUncheckedShadow = 0xFFFFU;
// Get the start address of the CFI shadow region.
uptr GetShadow() {
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return cfi_shadow_limits_storage.limits.start;
}
uptr GetShadowSize() {
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return cfi_shadow_limits_storage.limits.size;
}
// This will only work while the shadow is not allocated.
void SetShadowSize(uptr size) {
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cfi_shadow_limits_storage.limits.size = size;
}
uptr MemToShadowOffset(uptr x) {
return (x >> kShadowGranularity) << 1;
}
uint16_t *MemToShadow(uptr x, uptr shadow_base) {
return (uint16_t *)(shadow_base + MemToShadowOffset(x));
}
typedef int (*CFICheckFn)(u64, void *, void *);
// This class reads and decodes the shadow contents.
class ShadowValue {
uptr addr;
uint16_t v;
explicit ShadowValue(uptr addr, uint16_t v) : addr(addr), v(v) {}
public:
bool is_invalid() const { return v == kInvalidShadow; }
bool is_unchecked() const { return v == kUncheckedShadow; }
CFICheckFn get_cfi_check() const {
assert(!is_invalid() && !is_unchecked());
uptr aligned_addr = addr & ~(kShadowAlign - 1);
uptr p = aligned_addr - (((uptr)v - 1) << kShadowGranularity);
return reinterpret_cast<CFICheckFn>(p);
}
// Load a shadow value for the given application memory address.
static const ShadowValue load(uptr addr) {
uptr shadow_base = GetShadow();
uptr shadow_offset = MemToShadowOffset(addr);
if (shadow_offset > GetShadowSize())
return ShadowValue(addr, kInvalidShadow);
else
return ShadowValue(
addr, *reinterpret_cast<uint16_t *>(shadow_base + shadow_offset));
}
};
class ShadowBuilder {
uptr shadow_;
public:
// Allocate a new empty shadow (for the entire address space) on the side.
void Start();
// Mark the given address range as unchecked.
// This is used for uninstrumented libraries like libc.
// Any CFI check with a target in that range will pass.
void AddUnchecked(uptr begin, uptr end);
// Mark the given address range as belonging to a library with the given
// cfi_check function.
void Add(uptr begin, uptr end, uptr cfi_check);
// Finish shadow construction. Atomically switch the current active shadow
// region with the newly constructed one and deallocate the former.
void Install();
};
void ShadowBuilder::Start() {
shadow_ = (uptr)MmapNoReserveOrDie(GetShadowSize(), "CFI shadow");
VReport(1, "CFI: shadow at %zx .. %zx\n", shadow_, shadow_ + GetShadowSize());
}
void ShadowBuilder::AddUnchecked(uptr begin, uptr end) {
uint16_t *shadow_begin = MemToShadow(begin, shadow_);
uint16_t *shadow_end = MemToShadow(end - 1, shadow_) + 1;
// memset takes a byte, so our unchecked shadow value requires both bytes to
// be the same. Make sure we're ok during compilation.
static_assert((kUncheckedShadow & 0xff) == ((kUncheckedShadow >> 8) & 0xff),
"Both bytes of the 16-bit value must be the same!");
memset(shadow_begin, kUncheckedShadow & 0xff,
(shadow_end - shadow_begin) * sizeof(*shadow_begin));
}
void ShadowBuilder::Add(uptr begin, uptr end, uptr cfi_check) {
assert((cfi_check & (kShadowAlign - 1)) == 0);
// Don't fill anything below cfi_check. We can not represent those addresses
// in the shadow, and must make sure at codegen to place all valid call
// targets above cfi_check.
begin = Max(begin, cfi_check);
uint16_t *s = MemToShadow(begin, shadow_);
uint16_t *s_end = MemToShadow(end - 1, shadow_) + 1;
uint16_t sv = ((begin - cfi_check) >> kShadowGranularity) + 1;
for (; s < s_end; s++, sv++)
*s = sv;
}
#if SANITIZER_LINUX || SANITIZER_FREEBSD || SANITIZER_NETBSD
void ShadowBuilder::Install() {
MprotectReadOnly(shadow_, GetShadowSize());
uptr main_shadow = GetShadow();
if (main_shadow) {
// Update.
#if SANITIZER_LINUX
void *res = mremap((void *)shadow_, GetShadowSize(), GetShadowSize(),
MREMAP_MAYMOVE | MREMAP_FIXED, (void *)main_shadow);
CHECK(res != MAP_FAILED);
#elif SANITIZER_NETBSD
void *res = mremap((void *)shadow_, GetShadowSize(), (void *)main_shadow,
GetShadowSize(), MAP_FIXED);
CHECK(res != MAP_FAILED);
#else
void *res = MmapFixedOrDie(shadow_, GetShadowSize(), "cfi shadow");
CHECK(res != MAP_FAILED);
::memcpy(&shadow_, &main_shadow, GetShadowSize());
#endif
} else {
// Initial setup.
CHECK_EQ(kCfiShadowLimitsStorageSize, GetPageSizeCached());
CHECK_EQ(0, GetShadow());
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cfi_shadow_limits_storage.limits.start = shadow_;
MprotectReadOnly((uptr)&cfi_shadow_limits_storage,
sizeof(cfi_shadow_limits_storage));
CHECK_EQ(shadow_, GetShadow());
}
}
#else
#error not implemented
#endif
// This is a workaround for a glibc bug:
// https://sourceware.org/bugzilla/show_bug.cgi?id=15199
// Other platforms can, hopefully, just do
// dlopen(RTLD_NOLOAD | RTLD_LAZY)
// dlsym("__cfi_check").
uptr find_cfi_check_in_dso(dl_phdr_info *info) {
const Elf_Dyn *dynamic = nullptr;
for (int i = 0; i < info->dlpi_phnum; ++i) {
if (info->dlpi_phdr[i].p_type == PT_DYNAMIC) {
dynamic =
(const Elf_Dyn *)(info->dlpi_addr + info->dlpi_phdr[i].p_vaddr);
break;
}
}
if (!dynamic) return 0;
uptr strtab = 0, symtab = 0, strsz = 0;
for (const Elf_Dyn *p = dynamic; p->d_tag != PT_NULL; ++p) {
if (p->d_tag == DT_SYMTAB)
symtab = p->d_un.d_ptr;
else if (p->d_tag == DT_STRTAB)
strtab = p->d_un.d_ptr;
else if (p->d_tag == DT_STRSZ)
strsz = p->d_un.d_ptr;
}
if (symtab > strtab) {
VReport(1, "Can not handle: symtab > strtab (%zx > %zx)\n", symtab, strtab);
return 0;
}
// Verify that strtab and symtab are inside of the same LOAD segment.
// This excludes VDSO, which has (very high) bogus strtab and symtab pointers.
int phdr_idx;
for (phdr_idx = 0; phdr_idx < info->dlpi_phnum; phdr_idx++) {
const Elf_Phdr *phdr = &info->dlpi_phdr[phdr_idx];
if (phdr->p_type == PT_LOAD) {
uptr beg = info->dlpi_addr + phdr->p_vaddr;
uptr end = beg + phdr->p_memsz;
if (strtab >= beg && strtab + strsz < end && symtab >= beg &&
symtab < end)
break;
}
}
if (phdr_idx == info->dlpi_phnum) {
// Nope, either different segments or just bogus pointers.
// Can not handle this.
VReport(1, "Can not handle: symtab %zx, strtab %zx\n", symtab, strtab);
return 0;
}
for (const Elf_Sym *p = (const Elf_Sym *)symtab; (Elf_Addr)p < strtab;
++p) {
// There is no reliable way to find the end of the symbol table. In
// lld-produces files, there are other sections between symtab and strtab.
// Stop looking when the symbol name is not inside strtab.
if (p->st_name >= strsz) break;
char *name = (char*)(strtab + p->st_name);
if (strcmp(name, "__cfi_check") == 0) {
assert(p->st_info == ELF32_ST_INFO(STB_GLOBAL, STT_FUNC) ||
p->st_info == ELF32_ST_INFO(STB_WEAK, STT_FUNC));
uptr addr = info->dlpi_addr + p->st_value;
return addr;
}
}
return 0;
}
int dl_iterate_phdr_cb(dl_phdr_info *info, size_t size, void *data) {
uptr cfi_check = find_cfi_check_in_dso(info);
if (cfi_check)
VReport(1, "Module '%s' __cfi_check %zx\n", info->dlpi_name, cfi_check);
ShadowBuilder *b = reinterpret_cast<ShadowBuilder *>(data);
for (int i = 0; i < info->dlpi_phnum; i++) {
const Elf_Phdr *phdr = &info->dlpi_phdr[i];
if (phdr->p_type == PT_LOAD) {
// Jump tables are in the executable segment.
// VTables are in the non-executable one.
// Need to fill shadow for both.
// FIXME: reject writable if vtables are in the r/o segment. Depend on
// PT_RELRO?
uptr cur_beg = info->dlpi_addr + phdr->p_vaddr;
uptr cur_end = cur_beg + phdr->p_memsz;
if (cfi_check) {
VReport(1, " %zx .. %zx\n", cur_beg, cur_end);
b->Add(cur_beg, cur_end, cfi_check);
} else {
b->AddUnchecked(cur_beg, cur_end);
}
}
}
return 0;
}
// Init or update shadow for the current set of loaded libraries.
void UpdateShadow() {
ShadowBuilder b;
b.Start();
dl_iterate_phdr(dl_iterate_phdr_cb, &b);
b.Install();
}
void InitShadow() {
CHECK_EQ(0, GetShadow());
CHECK_EQ(0, GetShadowSize());
uptr vma = GetMaxUserVirtualAddress();
// Shadow is 2 -> 2**kShadowGranularity.
SetShadowSize((vma >> (kShadowGranularity - 1)) + 1);
VReport(1, "CFI: VMA size %zx, shadow size %zx\n", vma, GetShadowSize());
UpdateShadow();
}
THREADLOCAL int in_loader;
Mutex shadow_update_lock;
void EnterLoader() SANITIZER_NO_THREAD_SAFETY_ANALYSIS {
if (in_loader == 0) {
shadow_update_lock.Lock();
}
++in_loader;
}
void ExitLoader() SANITIZER_NO_THREAD_SAFETY_ANALYSIS {
CHECK(in_loader > 0);
--in_loader;
UpdateShadow();
if (in_loader == 0) {
shadow_update_lock.Unlock();
}
}
ALWAYS_INLINE void CfiSlowPathCommon(u64 CallSiteTypeId, void *Ptr,
void *DiagData) {
uptr Addr = (uptr)Ptr;
VReport(3, "__cfi_slowpath: %llx, %p\n", CallSiteTypeId, Ptr);
ShadowValue sv = ShadowValue::load(Addr);
if (sv.is_invalid()) {
VReport(1, "CFI: invalid memory region for a check target: %p\n", Ptr);
#ifdef CFI_ENABLE_DIAG
if (DiagData) {
__ubsan_handle_cfi_check_fail(
reinterpret_cast<__ubsan::CFICheckFailData *>(DiagData), Addr, false);
return;
}
#endif
Trap();
}
if (sv.is_unchecked()) {
VReport(2, "CFI: unchecked call (shadow=FFFF): %p\n", Ptr);
return;
}
CFICheckFn cfi_check = sv.get_cfi_check();
VReport(2, "__cfi_check at %p\n", (void *)cfi_check);
cfi_check(CallSiteTypeId, Ptr, DiagData);
}
void InitializeFlags() {
SetCommonFlagsDefaults();
#ifdef CFI_ENABLE_DIAG
__ubsan::Flags *uf = __ubsan::flags();
uf->SetDefaults();
#endif
FlagParser cfi_parser;
RegisterCommonFlags(&cfi_parser);
cfi_parser.ParseStringFromEnv("CFI_OPTIONS");
#ifdef CFI_ENABLE_DIAG
FlagParser ubsan_parser;
__ubsan::RegisterUbsanFlags(&ubsan_parser, uf);
RegisterCommonFlags(&ubsan_parser);
const char *ubsan_default_options = __ubsan_default_options();
ubsan_parser.ParseString(ubsan_default_options);
ubsan_parser.ParseStringFromEnv("UBSAN_OPTIONS");
#endif
InitializeCommonFlags();
if (Verbosity())
ReportUnrecognizedFlags();
if (common_flags()->help) {
cfi_parser.PrintFlagDescriptions();
}
}
} // namespace __cfi
using namespace __cfi;
extern "C" SANITIZER_INTERFACE_ATTRIBUTE void
__cfi_slowpath(u64 CallSiteTypeId, void *Ptr) {
CfiSlowPathCommon(CallSiteTypeId, Ptr, nullptr);
}
#ifdef CFI_ENABLE_DIAG
extern "C" SANITIZER_INTERFACE_ATTRIBUTE void
__cfi_slowpath_diag(u64 CallSiteTypeId, void *Ptr, void *DiagData) {
CfiSlowPathCommon(CallSiteTypeId, Ptr, DiagData);
}
#endif
static void EnsureInterceptorsInitialized();
// Setup shadow for dlopen()ed libraries.
// The actual shadow setup happens after dlopen() returns, which means that
// a library can not be a target of any CFI checks while its constructors are
// running. It's unclear how to fix this without some extra help from libc.
// In glibc, mmap inside dlopen is not interceptable.
// Maybe a seccomp-bpf filter?
// We could insert a high-priority constructor into the library, but that would
// not help with the uninstrumented libraries.
INTERCEPTOR(void*, dlopen, const char *filename, int flag) {
EnsureInterceptorsInitialized();
EnterLoader();
void *handle = REAL(dlopen)(filename, flag);
ExitLoader();
return handle;
}
INTERCEPTOR(int, dlclose, void *handle) {
EnsureInterceptorsInitialized();
EnterLoader();
int res = REAL(dlclose)(handle);
ExitLoader();
return res;
}
static Mutex interceptor_init_lock;
static bool interceptors_inited = false;
static void EnsureInterceptorsInitialized() {
Lock lock(&interceptor_init_lock);
if (interceptors_inited)
return;
INTERCEPT_FUNCTION(dlopen);
INTERCEPT_FUNCTION(dlclose);
interceptors_inited = true;
}
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extern "C" SANITIZER_INTERFACE_ATTRIBUTE
#if !SANITIZER_CAN_USE_PREINIT_ARRAY
// On ELF platforms, the constructor is invoked using .preinit_array (see below)
__attribute__((constructor(0)))
#endif
void __cfi_init() {
SanitizerToolName = "CFI";
InitializeFlags();
InitShadow();
#ifdef CFI_ENABLE_DIAG
__ubsan::InitAsPlugin();
#endif
}
#if SANITIZER_CAN_USE_PREINIT_ARRAY
// On ELF platforms, run cfi initialization before any other constructors.
// On other platforms we use the constructor attribute to arrange to run our
// initialization early.
extern "C" {
__attribute__((section(".preinit_array"),
used)) void (*__cfi_preinit)(void) = __cfi_init;
}
#endif