---
id: CVE-2026-89551
title: 'SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow'
summary: |-
  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…
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-191
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    a3d77bcd974b8625d16bddf448cb3a1b5e37049c
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    fa16bbe987b47e771e52eeb1b4540f18d92496ac
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    a924ac4c78afab71bf82641afa3b62e0c4a8b55e
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    e6267cccd7b05cc514e57f2160aa8db85f5c2701
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    ad0cce80d4af2f74674e8b635d97aa3880e83da8
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    85e9602650e9df07190abe817cee3b4d9bc3df17
  - >-
    Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 <
    3f491306dcb673ff5e78e1044ba450c58978774e
  - Linux 3.9
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-10-16T19:23:51.789342Z'
published: '2026-09-11'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T08:48:44.059Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-89551'
references:
  - url: 'https://git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b5e37049c'
  - url: 'https://git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18d92496ac'
  - url: 'https://git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0c4a8b55e'
  - url: 'https://git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4'
  - url: 'https://git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db85f5c2701'
  - url: 'https://git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa3880e83da8'
  - url: 'https://git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d9bc3df17'
  - url: 'https://git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c58978774e'
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89551.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-89551'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2532375'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-89551'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-89551'
  - url: >-
      https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89551.mbox
tags:
  - cve.org
  - csaf
  - vex
  - red-hat
  - score-dispute
epss: 0.00514
epssPercentile: 0.43947
scores:
  cna: 9.8
  vendor: 7
ingestedAt: '2026-09-14T15:23:07.452Z'
---

## Overview

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.

## Affected

- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < a3d77bcd974b8625d16bddf448cb3a1b5e37049c`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < fa16bbe987b47e771e52eeb1b4540f18d92496ac`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < a924ac4c78afab71bf82641afa3b62e0c4a8b55e`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 4bf59cb0ea5b0ddfbc46a1dc2fa78fc8b9986ce4`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < e6267cccd7b05cc514e57f2160aa8db85f5c2701`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < ad0cce80d4af2f74674e8b635d97aa3880e83da8`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 85e9602650e9df07190abe817cee3b4d9bc3df17`
- `Linux >= 4c190e2f913f038c9c91ee63b59cd037260ba353 < 3f491306dcb673ff5e78e1044ba450c58978774e`
- `Linux 3.9`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Vendor advisories

- **Red Hat VEX** · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-16 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89551.json)