CVE-2026-89542

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

SUNRPC: harden gss krb5 unwrap v2 against short tokens

gss krb5 unwrap v2() reads the EC and RRC header fields at ptr+4 and ptr+6 before validating that the token is at least GSS KRB5 TOK HDR LEN (16) bytes long, and its rotate left() helper passes buf->len - base to xdr buf subsegment() without verifying that base <= buf->len. When a caller hands in a sub-16-byte token, or a token whose declared len leaves base past the end of the buffer, three distinct failures follow:

gss krb5 unwrap v2(offset, len, buf) ptr = buf->head[0].iov base + offset ec = (ptr + 4) / OOB read on short head */ rrc = (ptr + 6) / OOB read on short head / rotate left(offset + 16, buf, rrc) xdr buf subsegment(buf, &subbuf, base, buf->len - base) / u32 wrap when base > len / rotate left(&subbuf, shift) shift %= buf->len / divide-by-zero when base == len */

After decryption, the cleanup arithmetic has the same shape:

movelen = min t(unsigned int, buf->head[0].iov len, len); movelen -= offset + GSS KRB5 TOK HDR LEN + headskip; BUG ON(offset + GSS KRB5 TOK HDR LEN + headskip + movelen > buf->head[0].iov len);

The BUG ON re-adds the value just subtracted, so it reduces to min(A, B) > A and is permanently false; it cannot catch the unsigned underflow of movelen, which then drives a ~UINT MAX-byte memmove().

Add four defense-in-depth guards inside the unwrap core so it is safe regardless of what its callers validate: