CVE-2025-38591

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 doesnt 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 didnt get reported to the mailing list.

Affected Software

References

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