liblzma: Don't verify header CRC32s if building for fuzz testing.

FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION is #defined when liblzma
is being built for fuzz testing.

Most fuzzed inputs would normally get rejected because of incorrect
CRC32 and the actual header decoding code wouldn't get fuzzed.
Disabling CRC32 checks avoids this problem. The fuzzer program
must still use LZMA_IGNORE_CHECK flag to disable verification of
integrity checks of uncompressed data.

src/liblzma/common/block_header_decoder.c

@@ -67,8 +67,11 @@ lzma_block_header_decode(lzma_block *block,
 	const size_t in_size = block->header_size - 4;
 
 	// Verify CRC32
-	if (lzma_crc32(in, in_size, 0) != unaligned_read32le(in + in_size))
+	if (lzma_crc32(in, in_size, 0) != unaligned_read32le(in + in_size)) {
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 		return LZMA_DATA_ERROR;
+#endif
+	}
 
 	// Check for unsupported flags.
 	if (in[1] & 0x3C)

src/liblzma/common/index_decoder.c

@@ -180,8 +180,11 @@ index_decode(void *coder_ptr, const lzma_allocator *allocator,
 				return LZMA_OK;
 
 			if (((coder->crc32 >> (coder->pos * 8)) & 0xFF)
-					!= in[(*in_pos)++])
+					!= in[(*in_pos)++]) {
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 				return LZMA_DATA_ERROR;
+#endif
+			}
 
 		} while (++coder->pos < 4);
 

src/liblzma/common/index_hash.c

@@ -313,8 +313,11 @@ lzma_index_hash_decode(lzma_index_hash *index_hash, const uint8_t *in,
 				return LZMA_OK;
 
 			if (((index_hash->crc32 >> (index_hash->pos * 8))
-					& 0xFF) != in[(*in_pos)++])
+					& 0xFF) != in[(*in_pos)++]) {
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 				return LZMA_DATA_ERROR;
+#endif
+			}
 
 		} while (++index_hash->pos < 4);
 

src/liblzma/common/stream_flags_decoder.c

@@ -39,8 +39,11 @@ lzma_stream_header_decode(lzma_stream_flags *options, const uint8_t *in)
 	const uint32_t crc = lzma_crc32(in + sizeof(lzma_header_magic),
 			LZMA_STREAM_FLAGS_SIZE, 0);
 	if (crc != unaligned_read32le(in + sizeof(lzma_header_magic)
-			+ LZMA_STREAM_FLAGS_SIZE))
+			+ LZMA_STREAM_FLAGS_SIZE)) {
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 		return LZMA_DATA_ERROR;
+#endif
+	}
 
 	// Stream Flags
 	if (stream_flags_decode(options, in + sizeof(lzma_header_magic)))
@@ -67,8 +70,11 @@ lzma_stream_footer_decode(lzma_stream_flags *options, const uint8_t *in)
 	// CRC32
 	const uint32_t crc = lzma_crc32(in + sizeof(uint32_t),
 			sizeof(uint32_t) + LZMA_STREAM_FLAGS_SIZE, 0);
-	if (crc != unaligned_read32le(in))
+	if (crc != unaligned_read32le(in)) {
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 		return LZMA_DATA_ERROR;
+#endif
+	}
 
 	// Stream Flags
 	if (stream_flags_decode(options, in + sizeof(uint32_t) * 2))