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executor: protect against memory corruptions better

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Fuzzer has figured out how to corrupt input/output shmem regions
abusing the text memcpy in syz_kvm_setup_cpu. It guessed a negative
text_size value that causes the memcpy to overwrite shmem regions.
Protect better against such cases:
1. Make text_size unsigned (there is already a check that it is less than 1000).
2. Map input region as readable only, we don't write to it.
3. Add address sanity check to segv_handler, if we see that we are writing
   into executable data, it's better to crash instantly.
This commit is contained in:
Dmitry Vyukov 2017-01-25 11:01:30 +01:00
parent 40723a067e
commit f810d08444
4 changed files with 23 additions and 5 deletions
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10
csource/common.go
View File

@ -116,8 +116,14 @@ __thread jmp_buf segv_env;
static void segv_handler(int sig, siginfo_t* info, void* uctx)
{
if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED))
uintptr_t addr = (uintptr_t)info->si_addr;
const uintptr_t prog_start = 1<<20;
const uintptr_t prog_end = 100<<20;
if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED) && (addr < prog_start || addr > prog_end)) {
debug("SIGSEGV on %p, skipping\n", addr);
_longjmp(segv_env, 1);
}
debug("SIGSEGV on %p, exiting\n", addr);
doexit(sig);
for (;;) {
}
@ -711,7 +717,7 @@ static uintptr_t syz_kvm_setup_cpu(uintptr_t a0, uintptr_t a1, uintptr_t a2, uin
(void)text_count;
int text_type = 0;
const void* text = 0;
int text_size = 0;
uintptr_t text_size = 0;
NONFAILING(text_type = text_array_ptr[0].typ);
NONFAILING(text = text_array_ptr[0].text);
NONFAILING(text_size = text_array_ptr[0].size);

14
executor/common.h
View File

@ -132,8 +132,20 @@ __thread jmp_buf segv_env;
static void segv_handler(int sig, siginfo_t* info, void* uctx)
{
if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED))
// Generated programs can contain bad (unmapped/protected) addresses,
// which cause SIGSEGVs during copyin/copyout.
// This handler ignores such crashes to allow the program to proceed.
// We additionally opportunistically check that the faulty address
// is not within executable data region, because such accesses can corrupt
// output region and then fuzzer will fail on corrupted data.
uintptr_t addr = (uintptr_t)info->si_addr;
const uintptr_t prog_start = 1 << 20;
const uintptr_t prog_end = 100 << 20;
if (__atomic_load_n(&skip_segv, __ATOMIC_RELAXED) && (addr < prog_start || addr > prog_end)) {
debug("SIGSEGV on %p, skipping\n", addr);
_longjmp(segv_env, 1);
}
debug("SIGSEGV on %p, exiting\n", addr);
doexit(sig);
for (;;) {
}

2
executor/common_kvm_amd64.h
View File

@ -287,7 +287,7 @@ static uintptr_t syz_kvm_setup_cpu(uintptr_t a0, uintptr_t a1, uintptr_t a2, uin
(void)text_count; // fuzzer can spoof count and we need just 1 text, so ignore text_count
int text_type = 0;
const void* text = 0;
int text_size = 0;
uintptr_t text_size = 0;
NONFAILING(text_type = text_array_ptr[0].typ);
NONFAILING(text = text_array_ptr[0].text);
NONFAILING(text_size = text_array_ptr[0].size);

2
executor/executor.cc
View File

@ -139,7 +139,7 @@ int main(int argc, char** argv)
}
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
if (mmap(&input_data[0], kMaxInput, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_FIXED, kInFd, 0) != &input_data[0])
if (mmap(&input_data[0], kMaxInput, PROT_READ, MAP_PRIVATE | MAP_FIXED, kInFd, 0) != &input_data[0])
fail("mmap of input file failed");
if (mmap(&output_data[0], kMaxOutput, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, kOutFd, 0) != &output_data[0])
fail("mmap of output file failed");