CVE-2026-63886: scsi: target: iscsi: Validate CHAP_R length before base64 decode

In the Linux kernel, the following vulnerability has been resolved:

scsi: target: iscsi: Validate CHAPR length before base64 decode

chapservercomputehash() allocates clientdigest as kzalloc(chap->digestsize) and then, for BASE64-encoded responses, passes chapr directly to chapbase64decode() without checking whether the input length could produce more than digestsize bytes of output.

chapbase64decode() writes to the destination unconditionally as long as there is input to consume. With MAXRESPONSELENGTH set to 128 and the "0b" prefix stripped by extractparam(), up to 127 base64 characters can reach the decoder. 127 characters decode to 95 bytes. For SHA-256 (digestsize=32) this overflows clientdigest by 63 bytes; for MD5 (digestsize=16) the overflow is 79 bytes.

The length check at line 344 fires after the write has already happened.

The HEX branch in the same switch statement already validates the length up front. Apply the same approach to the BASE64 branch: strip trailing base64 padding characters, then reject any input whose data length exceeds DIVROUNDUP(digestsize 4, 3) before calling the decoder.

Stripping trailing '=' before the comparison handles both padded and unpadded encodings. chapbase64decode() already returns early on '=', so the full original string is still passed to the decoder unchanged.

The mutual CHAP path decodes CHAPC into initiatorchgbinhex, which is kzalloc(CHAPCHALLENGESTRLEN). extractparam() caps initiatorchg at CHAPCHALLENGESTRLEN characters, so at most CHAPCHALLENGESTRLEN-1 base64 characters reach the decoder. The maximum decoded size, DIVROUNDUP((CHAPCHALLENGESTRLEN-1) 3, 4), is less than CHAPCHALLENGESTRLEN, so no overflow is possible there. A comment is added at the call site to document this.