linux-cve-announce - CVE-2025-40123: bpf: Enforce expected_attach

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Message-ID: <2025111253-CVE-2025-40123-fcb1@gregkh>
Date: Wed, 12 Nov 2025 19:23:59 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40123: bpf: Enforce expected_attach_type for tailcall compatibility

From: Greg Kroah-Hartman <gregkh@...nel.org>

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.

The Linux kernel CVE team has assigned CVE-2025-40123 to this issue.

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

	Issue introduced in 4.17 with commit 5e43f899b03a3492ce5fc44e8900becb04dae9c0 and fixed in 6.1.156 with commit a99de19128aec0913f3d529f529fbbff5edfaff8
	Issue introduced in 4.17 with commit 5e43f899b03a3492ce5fc44e8900becb04dae9c0 and fixed in 6.6.112 with commit 08cb3dc9d2b44f153d0bcf2cb966e4a94b5d0f32
	Issue introduced in 4.17 with commit 5e43f899b03a3492ce5fc44e8900becb04dae9c0 and fixed in 6.12.53 with commit f856c598080ba7ce1252867b8ecd6ad5bdaf9a6a
	Issue introduced in 4.17 with commit 5e43f899b03a3492ce5fc44e8900becb04dae9c0 and fixed in 6.17.3 with commit c1ad19b5d8e23123503dcaf2d4342e1b90b923ad
	Issue introduced in 4.17 with commit 5e43f899b03a3492ce5fc44e8900becb04dae9c0 and fixed in 6.18-rc1 with commit 4540aed51b12bc13364149bf95f6ecef013197c0

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-2025-40123
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:
	include/linux/bpf.h
	kernel/bpf/core.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/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