UBUNTU-CVE-2026-89538: Critical severity ubuntu/linux vulnerability

In the Linux kernel, the following vulnerability has been resolved: SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field gsskrb5unwrapv2() sets buf->len to a logical length, which can be much smaller than head[0].iovlen (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 GSSSDEFECTIVETOKEN error and keeping the xdrbuf internally consistent. The wrapped blob begins at a nonzero offset -- both callers pass len as offset + opaquelen -- 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.