Headline
CVE-2020-22219: wild-addr-write found by fuzz · Issue #215 · xiph/flac
Buffer Overflow vulnerability in function bitwriter_grow_ in flac before 1.4.0 allows remote attackers to run arbitrary code via crafted input to the encoder.
we found wild-addr-write by fuzzing flac-master:
==217==ERROR: AddressSanitizer: SEGV on unknown address 0xb6029a2c (pc 0x0822a2ae bp 0xffeb31e8 sp 0xffeb30a0 T0)
==217==The signal is caused by a WRITE memory access.
SCARINESS: 30 (wild-addr-write)
#0 0x822a2ad in FLAC__bitwriter_write_raw_uint32_nocheck /src/flac/src/libFLAC/bitwriter.c
#1 0x8229a42 in FLAC__bitwriter_write_raw_uint32 /src/flac/src/libFLAC/bitwriter.c:369:9
#2 0x8218ec3 in FLAC__frame_add_header /src/flac/src/libFLAC/stream_encoder_framing.c:227:6
#3 0x820557b in process_subframes_ /src/flac/src/libFLAC/stream_encoder.c:3365:7
#4 0x81d940f in process_frame_ /src/flac/src/libFLAC/stream_encoder.c:3096:6
#5 0x81f3770 in FLAC__stream_encoder_process_interleaved /src/flac/src/libFLAC/stream_encoder.c:2298:9
#6 0x81bfa80 in FLAC::Encoder::Stream::process_interleaved(int const*, unsigned int) /src/flac/src/libFLAC++/stream_encoder.cpp:370:29
#7 0x81ac167 in LLVMFuzzerTestOneInput /src/flac-fuzzers/fuzzer_encoder.cpp:141:46
#8 0x80ac766 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned int) /src/llvm/projects/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:556:15
#9 0x8098c13 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned int) /src/llvm/projects/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:292:6
#10 0x809e318 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned int)) /src/llvm/projects/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:774:9
#11 0x80c3167 in main /src/llvm/projects/compiler-rt/lib/fuzzer/FuzzerMain.cpp:19:10
#12 0xf7539636 in __libc_start_main (/lib32/libc.so.6+0x18636)
#13 0x8073c38 in _start (/out/flac/fuzzer_encoder+0x8073c38)
here is my debug info:
bw->buffer was realloc here
bitwriter_grow_ (bw=0xf5a00a90, bits_to_add=62914562) at bitwriter.c:128
128 if(new_buffer == 0)
(gdb) n
130 bw->buffer = new_buffer;
(gdb) l
125 FLAC__ASSERT(new_capacity >= bw->words + ((bw->bits + bits_to_add + FLAC__BITS_PER_WORD - 1) / FLAC__BITS_PER_WORD));
126
127 new_buffer = safe_realloc_mul_2op_(bw->buffer, sizeof(bwword), /*times*/new_capacity);
128 if(new_buffer == 0)
129 return false;
130 bw->buffer = new_buffer;
131 bw->capacity = new_capacity;
132 return true;
133 }
134
(gdb) p new_buffer
$1 = (bwword *) 0x7abd7800
(gdb) p new_capacity
$2 = 250956800
later, bw->buffer was freed but it’s value NOT set to 0
156 static inline void *safe_realloc_(void *ptr, size_t size)
157 {
158 void *oldptr = ptr;
159 void *newptr = realloc(ptr, size);
160 if(size > 0 && newptr == 0)
161 free(oldptr);
162 return newptr;
(gdb) n
159 void *newptr = realloc(ptr, size);
(gdb) n
160 if(size > 0 && newptr == 0)
(gdb) p newptr
$4 = (void *) 0x0
(gdb) p size
$5 = 1006448640
(gdb) n
161 free(oldptr);
(gdb) p oldptr
$6 = (void *) 0x7abd7800
(gdb) n
162 return newptr;
(gdb) n
safe_realloc_mul_2op_ (ptr=0x7abd7800, size1=4, size2=251612160) at ../../include/share/alloc.h:206
206 }
(gdb) n
bitwriter_grow_ (bw=0xf5a00a90, bits_to_add=20971521) at bitwriter.c:128
128 if(new_buffer == 0)
(gdb) l
123 FLAC__ASSERT(0 == (new_capacity - bw->capacity) % FLAC__BITWRITER_DEFAULT_INCREMENT);
124 FLAC__ASSERT(new_capacity > bw->capacity);
125 FLAC__ASSERT(new_capacity >= bw->words + ((bw->bits + bits_to_add + FLAC__BITS_PER_WORD - 1) / FLAC__BITS_PER_WORD));
126
127 new_buffer = safe_realloc_mul_2op_(bw->buffer, sizeof(bwword), /*times*/new_capacity);
128 if(new_buffer == 0)
129 return false;
130 bw->buffer = new_buffer;
131 bw->capacity = new_capacity;
132 return true;
(gdb) p bw->buffer
$7 = (bwword *) 0x7abd7800
(gdb) p bw->capacity
$8 = 250956800
(gdb) n
129 return falseRelated news
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