CVE-2026-89538

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

SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

gsskrb5unwrap_v2() sets buf->len to a logical

length, which can be much smaller than head[0].iov_len

(the allocated receive-page capacity).  It then calls

xdrbuftrim() with a trim length derived from the 16-bit

"extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp()

against the encrypted header copy, so a randomly-mutated

value is rejected.  However, any peer holding a valid GSS

context can legitimately encrypt a token whose ec exceeds

the plaintext length.  Per RFC 4121, such a token is

structurally malformed.

Although xdrbuftrim() now clamps the buf->len subtraction

to avoid unsigned underflow, the buffer is still left in a

semantically invalid state (zero length, inconsistent iov

lengths) when ec is oversized.

Reject these tokens before calling xdrbuftrim(), giving

callers a well-defined GSSSDEFECTIVE_TOKEN error and

keeping the xdr_buf internally consistent.  The wrapped blob

begins at a nonzero offset -- both callers pass len as

offset + opaque_len -- so buf->len still counts the offset

bytes that precede the blob.  Compare the trim length

against the remaining wrapped segment, buf->len - offset,

rather than the whole buffer; comparing against buf->len

alone leaves an offset-wide window in which an oversized ec

passes the test and xdrbuftrim() cuts into the bytes ahead

of the blob.