linux-cve-announce - CVE-2025-38591: bpf: Reject narrower access to pointer ctx fields

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Message-ID: <2025081917-CVE-2025-38591-2a4d@gregkh>
Date: Tue, 19 Aug 2025 19:18:37 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38591: bpf: Reject narrower access to pointer ctx fields

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

Description
===========

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

bpf: Reject narrower access to pointer ctx fields

The following BPF program, simplified from a syzkaller repro, causes a
kernel warning:

    r0 = *(u8 *)(r1 + 169);
    exit;

With pointer field sk being at offset 168 in __sk_buff. This access is
detected as a narrower read in bpf_skb_is_valid_access because it
doesn't match offsetof(struct __sk_buff, sk). It is therefore allowed
and later proceeds to bpf_convert_ctx_access. Note that for the
"is_narrower_load" case in the convert_ctx_accesses(), the insn->off
is aligned, so the cnt may not be 0 because it matches the
offsetof(struct __sk_buff, sk) in the bpf_convert_ctx_access. However,
the target_size stays 0 and the verifier errors with a kernel warning:

    verifier bug: error during ctx access conversion(1)

This patch fixes that to return a proper "invalid bpf_context access
off=X size=Y" error on the load instruction.

The same issue affects multiple other fields in context structures that
allow narrow access. Some other non-affected fields (for sk_msg,
sk_lookup, and sockopt) were also changed to use bpf_ctx_range_ptr for
consistency.

Note this syzkaller crash was reported in the "Closes" link below, which
used to be about a different bug, fixed in
commit fce7bd8e385a ("bpf/verifier: Handle BPF_LOAD_ACQ instructions
in insn_def_regno()"). Because syzbot somehow confused the two bugs,
the new crash and repro didn't get reported to the mailing list.

The Linux kernel CVE team has assigned CVE-2025-38591 to this issue.

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

	Issue introduced in 4.13 with commit f96da09473b52c09125cc9bf7d7d4576ae8229e0 and fixed in 6.16.1 with commit 202900ceeef67458c964c2af6e1427c8e533ea7c
	Issue introduced in 4.13 with commit f96da09473b52c09125cc9bf7d7d4576ae8229e0 and fixed in 6.17-rc1 with commit e09299225d5ba3916c91ef70565f7d2187e4cca0

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-2025-38591
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:
	kernel/bpf/cgroup.c
	net/core/filter.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/202900ceeef67458c964c2af6e1427c8e533ea7c
	https://git.kernel.org/stable/c/e09299225d5ba3916c91ef70565f7d2187e4cca0