lib/decompress: check for reserved bit corruption in zstd

The patch adds a validation to ensure that the last field, which is
reserved, must be all-zeroes in ZSTD_decodeSeqHeaders. This prevents
potential corruption from going undetected.

Fixes an issue where corrupted input could lead to undefined behavior
due to improper validation of reserved bits.

Signed-off-by: aimuz <mr.imuz@gmail.com>

Author: aimuz

lib/decompress/zstd_decompress_block.c

@@ -607,6 +607,7 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
 
     /* FSE table descriptors */
     RETURN_ERROR_IF(ip+1 > iend, srcSize_wrong, ""); /* minimum possible size: 1 byte for symbol encoding types */
+    RETURN_ERROR_IF(*ip & 3, corruption_detected, ""); /* The last field, Reserved, must be all-zeroes. */
     {   symbolEncodingType_e const LLtype = (symbolEncodingType_e)(*ip >> 6);
         symbolEncodingType_e const OFtype = (symbolEncodingType_e)((*ip >> 4) & 3);
         symbolEncodingType_e const MLtype = (symbolEncodingType_e)((*ip >> 2) & 3);