CVE-2026-89551: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow

[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: xdr_buf_trim: clamp buf->len to avoid underflow

xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by
walking the tail, pages, and head iovecs.  Each per-section step
uses min_t() so it never removes more bytes than that section
holds, but the final accounting at the fix_len label subtracts the
total bytes actually consumed from buf->len without any clamp:

    fix_len:
            buf->len -= (len - trim);

When the caller has set buf->len to a value smaller than the sum
of the iov_lens, (len - trim) can exceed buf->len and the unsigned
subtraction wraps to near UINT_MAX.  gss_krb5_unwrap_v2() reaches
xdr_buf_trim() in exactly that state:

    buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
    buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);
    xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);

buf->len is a small wire-derived value while the iov_lens are at
page scale, so the per-section loops legitimately consume far more
bytes than buf->len records.  The wrapped buf->len then propagates
as the authoritative stream bound into every downstream XDR
decoder.

Fix by clamping the decrement so buf->len bottoms out at zero:

    buf->len -= min_t(unsigned int, buf->len, len - trim);

On the normal path where the iov_lens sum to buf->len, (len - trim)
is always <= buf->len and the result is identical to before.  No
callers change behavior outside the underflow case.

The Linux kernel CVE team has assigned CVE-2026-89551 to this issue.


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

	Issue introduced in 3.9 with commit 4c190e2f913f038c9c91ee63b59cd037260ba353 and fixed in 6.12.109 with commit e6267cccd7b05cc514e57f2160aa8db85f5c2701
	Issue introduced in 3.9 with commit 4c190e2f913f038c9c91ee63b59cd037260ba353 and fixed in 6.18.50 with commit ad0cce80d4af2f74674e8b635d97aa3880e83da8
	Issue introduced in 3.9 with commit 4c190e2f913f038c9c91ee63b59cd037260ba353 and fixed in 7.2.4 with commit 85e9602650e9df07190abe817cee3b4d9bc3df17
	Issue introduced in 3.9 with commit 4c190e2f913f038c9c91ee63b59cd037260ba353 and fixed in 7.3-rc1 with commit 3f491306dcb673ff5e78e1044ba450c58978774e

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-2026-89551
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/xdr.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/e6267cccd7b05cc514e57f2160aa8db85f5c2701
	https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8
	https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17
	https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e