---
id: CVE-2025-40123
title: 'bpf: Enforce expected_attach_type for tailcall compatibility'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  bpf: Enforce expected_attach_type for tailcall compatibility

  Yinhao et al. recently reported:

    Our fuzzer tool discovered an uninitialized pointer issue in the
    bpf…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    e53a59e7cee5c1605d1ed595933098c2b69108f9
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    dc4f348952173beb8ceb15be30b249276eccb605
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    a99de19128aec0913f3d529f529fbbff5edfaff8
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    c1ad19b5d8e23123503dcaf2d4342e1b90b923ad
  - >-
    Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 <
    4540aed51b12bc13364149bf95f6ecef013197c0
  - Linux 4.17
exploitAvailable: true
published: '2025-11-12'
updated: '2026-09-14'
sourceUpdated: '2026-09-14T11:58:08.635Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-40123'
references:
  - url: 'https://git.kernel.org/stable/c/e53a59e7cee5c1605d1ed595933098c2b69108f9'
  - url: 'https://git.kernel.org/stable/c/dc4f348952173beb8ceb15be30b249276eccb605'
  - url: 'https://git.kernel.org/stable/c/a99de19128aec0913f3d529f529fbbff5edfaff8'
  - url: 'https://git.kernel.org/stable/c/08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32'
  - url: 'https://git.kernel.org/stable/c/f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a'
  - url: 'https://git.kernel.org/stable/c/c1ad19b5d8e23123503dcaf2d4342e1b90b923ad'
  - url: 'https://git.kernel.org/stable/c/4540aed51b12bc13364149bf95f6ecef013197c0'
tags:
  - cve.org
  - exploit-available
epss: 0.00155
epssPercentile: 0.03882
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/gduma-phData/patch-CVE-2025-40123'
  checkedAt: '2026-09-26T09:05:33.477Z'
ingestedAt: '2026-09-14T15:23:07.459Z'
---

## Overview

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

bpf: Enforce expected_attach_type for tailcall compatibility

Yinhao et al. recently reported:

  Our fuzzer tool discovered an uninitialized pointer issue in the
  bpf_prog_test_run_xdp() function within the Linux kernel's BPF subsystem.
  This leads to a NULL pointer dereference when a BPF program attempts to
  deference the txq member of struct xdp_buff object.

The test initializes two programs of BPF_PROG_TYPE_XDP: progA acts as the
entry point for bpf_prog_test_run_xdp() and its expected_attach_type can
neither be of be BPF_XDP_DEVMAP nor BPF_XDP_CPUMAP. progA calls into a slot
of a tailcall map it owns. progB's expected_attach_type must be BPF_XDP_DEVMAP
to pass xdp_is_valid_access() validation. The program returns struct xdp_md's
egress_ifindex, and the latter is only allowed to be accessed under mentioned
expected_attach_type. progB is then inserted into the tailcall which progA
calls.

The underlying issue goes beyond XDP though. Another example are programs
of type BPF_PROG_TYPE_CGROUP_SOCK_ADDR. sock_addr_is_valid_access() as well
as sock_addr_func_proto() have different logic depending on the programs'
expected_attach_type. Similarly, a program attached to BPF_CGROUP_INET4_GETPEERNAME
should not be allowed doing a tailcall into a program which calls bpf_bind()
out of BPF which is only enabled for BPF_CGROUP_INET4_CONNECT.

In short, specifying expected_attach_type allows to open up additional
functionality or restrictions beyond what the basic bpf_prog_type enables.
The use of tailcalls must not violate these constraints. Fix it by enforcing
expected_attach_type in __bpf_prog_map_compatible().

Note that we only enforce this for tailcall maps, but not for BPF devmaps or
cpumaps: There, the programs are invoked through dev_map_bpf_prog_run*() and
cpu_map_bpf_prog_run*() which set up a new environment / context and therefore
these situations are not prone to this issue.

## Affected

- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < e53a59e7cee5c1605d1ed595933098c2b69108f9`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < dc4f348952173beb8ceb15be30b249276eccb605`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < a99de19128aec0913f3d529f529fbbff5edfaff8`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < 08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < c1ad19b5d8e23123503dcaf2d4342e1b90b923ad`
- `Linux >= 5e43f899b03a3492ce5fc44e8900becb04dae9c0 < 4540aed51b12bc13364149bf95f6ecef013197c0`
- `Linux 4.17`

## Remediation

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