CVE-2026-89538: SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

gss_krb5_unwrap_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
xdr_buf_trim() 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 xdr_buf_trim() 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 xdr_buf_trim(), giving
callers a well-defined GSS_S_DEFECTIVE_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 xdr_buf_trim() cuts into the bytes ahead
of the blob.

The Linux kernel CVE team has assigned CVE-2026-89538 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.13 with commit cf4c024b908353fcc48309374d39e3399d67dfd1 and fixed in 6.12.109 with commit dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
	Issue introduced in 3.13 with commit cf4c024b908353fcc48309374d39e3399d67dfd1 and fixed in 6.18.50 with commit 1f9856af065b6158271687e9788407b9573fd16f
	Issue introduced in 3.13 with commit cf4c024b908353fcc48309374d39e3399d67dfd1 and fixed in 7.2.4 with commit 880effc943ed82d6811f272ed3307c222b40f4d6
	Issue introduced in 3.13 with commit cf4c024b908353fcc48309374d39e3399d67dfd1 and fixed in 7.3-rc1 with commit ad484748eec0a66eac0f13ab53b3fbedb7333c91

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89538
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/sunrpc/auth_gss/gss_krb5_wrap.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
	https://git.kernel.org/stable/c/1f9856af065b6158271687e9788407b9573fd16f
	https://git.kernel.org/stable/c/880effc943ed82d6811f272ed3307c222b40f4d6
	https://git.kernel.org/stable/c/ad484748eec0a66eac0f13ab53b3fbedb7333c91