CVE-2025-38591 Information

Aug 20, 2025 cve

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 (pf/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.

Reference

https://git.kernel.org/stable/c/202900ceeef67458c964c2af6e1427c8e533ea7c https://git.kernel.org/stable/c/e09299225d5ba3916c91ef70565f7d2187e4cca0

CNNVD-202508-2170 (Published: 2025-08-19)

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