DEBIAN-CVE-2026-89538

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 xdrbuf_trim() cuts into the bytes ahead of the blob.