---
id: CVE-2026-80557
title: 'libceph: fix OOB read in decode_watchers() via missing bounds check'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  libceph: fix OOB read in decode_watchers() via missing bounds check

  ceph_start_decoding() validates that struct_len bytes remain in the
  buffer after the encoding heade…
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
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    85479b7d65b4ebcb07fbbe57230976793974ab4a
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    1c824e7c75bb4adf19553dd4ea944a5d83096be8
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    f161be39201eb5f9b1f58fb8f90b8a9cd3931eb6
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    eab3eeb68bfc639d74f27256f05546af5c4f787d
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    c59219a6b62d74936963983e5815524c3de8dd79
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    cb8246e5846dbbe34930903a90c7a90dd8e5910b
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1
  - >-
    Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c <
    00ead17c7de137a692edee59f2772e6af687e8eb
  - Linux 4.9
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-10-16T19:17:25.527528Z'
published: '2026-08-26'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T19:26:26.946Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-80557'
references:
  - url: 'https://git.kernel.org/stable/c/85479b7d65b4ebcb07fbbe57230976793974ab4a'
  - url: 'https://git.kernel.org/stable/c/1c824e7c75bb4adf19553dd4ea944a5d83096be8'
  - url: 'https://git.kernel.org/stable/c/f161be39201eb5f9b1f58fb8f90b8a9cd3931eb6'
  - url: 'https://git.kernel.org/stable/c/eab3eeb68bfc639d74f27256f05546af5c4f787d'
  - url: 'https://git.kernel.org/stable/c/c59219a6b62d74936963983e5815524c3de8dd79'
  - url: 'https://git.kernel.org/stable/c/cb8246e5846dbbe34930903a90c7a90dd8e5910b'
  - url: 'https://git.kernel.org/stable/c/7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1'
  - url: 'https://git.kernel.org/stable/c/00ead17c7de137a692edee59f2772e6af687e8eb'
tags:
  - cve.org
epss: 0.00525
epssPercentile: 0.43244
ingestedAt: '2026-10-16T19:46:24.315Z'
---

## Overview

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

libceph: fix OOB read in decode_watchers() via missing bounds check

ceph_start_decoding() validates that struct_len bytes remain in the
buffer after the encoding header, but accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends an obj_list_watch_response_t reply with
struct_len=0, ceph_start_decoding() returns success with p == end,
leaving zero bytes guaranteed for subsequent reads.

The immediately following ceph_decode_32(p) in decode_watchers() has
no preceding bounds check. With p == end this is a 4-byte read past
the validated buffer boundary. The garbage value is then passed
directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants
(ceph_decode_copy_safe, ceph_decode_64_safe, ceph_decode_skip_32)
after its own ceph_start_decoding() call. decode_watchers() is the
only site that uses the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_watchers, bad), consistent with the established pattern.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment (e.g. cloud) can trigger this against any kernel client
that calls CEPH_OSD_OP_LIST_WATCHERS, without any further privileges
beyond OSD session establishment.

[ idryomov: trim changelog ]

## Affected

- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < 85479b7d65b4ebcb07fbbe57230976793974ab4a`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < 1c824e7c75bb4adf19553dd4ea944a5d83096be8`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < f161be39201eb5f9b1f58fb8f90b8a9cd3931eb6`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < eab3eeb68bfc639d74f27256f05546af5c4f787d`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < c59219a6b62d74936963983e5815524c3de8dd79`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < cb8246e5846dbbe34930903a90c7a90dd8e5910b`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < 7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1`
- `Linux >= a4ed38d7a180f184a6e7aedd09db9ca4b1e6a71c < 00ead17c7de137a692edee59f2772e6af687e8eb`
- `Linux 4.9`

## Remediation

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