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fixed bug 44168

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discovered by oss-fuzz

It's a bug in the test itself :
ZSTD_compressBound() as an upper bound of the compress size
only works for data compressed "normally".
But in situations where many flushes are forcefully introduced,
this creates many more blocks,
each of which has a potential to increase the size by 3 bytes.
In extreme cases (lots of small incompressible blocks), the expansion can go beyond ZSTD_compressBound().

This situation is similar when using the CompressSequences() API
with Explicit Block Delimiters.
In which case, each explicit block acts like a deliberate flush.
When employed by a fuzzer, it's possible to generate scenarios like the one described above,
with tons of incompressible blocks of small sizes,
thus going beyond ZSTD_compressBound().

fix : when using Explicit Block Delimiters, use a larger bound, to account for this scenario.
This commit is contained in:
Yann Collet 2022-01-29 16:23:21 -08:00
parent c9072dd60a
commit 637b2d7a24
3 changed files with 8 additions and 4 deletions
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4
lib/common/zstd_internal.h
View File

@ -311,8 +311,8 @@ typedef struct {
* in the seqStore that has a value larger than U16 (if it exists). To do so, we increment
* the existing value of the litLength or matchLength by 0x10000.
*/
ZSTD_longLengthType_e longLengthType;
U32 longLengthPos; /* Index of the sequence to apply long length modification to */
ZSTD_longLengthType_e longLengthType;
U32 longLengthPos; /* Index of the sequence to apply long length modification to */
} seqStore_t;
typedef struct {

2
lib/compress/zstd_compress.c
View File

@ -6140,7 +6140,7 @@ ZSTD_compressSequences_internal(ZSTD_CCtx* cctx,
FORWARD_IF_ERROR(blockSize, "Error while trying to determine block size");
assert(blockSize <= remaining);
ZSTD_resetSeqStore(&cctx->seqStore);
DEBUGLOG(5, "Working on new block. Blocksize: %zu", blockSize);
DEBUGLOG(5, "Working on new block. Blocksize: %zu (total:%zu)", blockSize, (ip - (const BYTE*)src) + blockSize);
additionalByteAdjustment = sequenceCopier(cctx, &seqPos, inSeqs, inSeqsSize, ip, blockSize);
FORWARD_IF_ERROR(additionalByteAdjustment, "Bad sequence copy");

6
tests/fuzz/sequence_compression_api.c
View File

@ -297,7 +297,11 @@ int LLVMFuzzerTestOneInput(const uint8_t* src, size_t size)
}
nbSequences = generateRandomSequences(producer, ZSTD_FUZZ_GENERATED_LITERALS_SIZE, dictSize, wLog, mode);
generatedSrcSize = decodeSequences(generatedSrc, nbSequences, ZSTD_FUZZ_GENERATED_LITERALS_SIZE, dictBuffer, dictSize, mode);
cBufSize = ZSTD_compressBound(generatedSrcSize);
/* Note : in explicit block delimiters mode,
* the fuzzer might generate a lot of small incompressible blocks.
* In which case, the final compressed size might be > ZSTD_compressBound().
* Solution : provide a much more generous cBufSize to cover these scenarios */
cBufSize = (mode == ZSTD_sf_noBlockDelimiters) ? ZSTD_compressBound(generatedSrcSize) : 256 + (generatedSrcSize * 2);
cBuf = FUZZ_malloc(cBufSize);
rBufSize = generatedSrcSize;