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 __skbuff. This access is detected as a narrower read in bpfskb_isvalidaccess because it doesn't match offsetof(struct __skbuff, sk). It is therefore allowed and later proceeds to bpfconvertctxaccess. Note that for the "isnarrowerload" case in the convertctxaccesses(), the insn->off is aligned, so the cnt may not be 0 because it matches the offsetof(struct _skbuff, sk) in the bpfconvertctxaccess. However, the targetsize 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 skmsg, sklookup, and sockopt) were also changed to use bpfctxrange_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 BPFLOADACQ instructions in insndefregno()"). Because syzbot somehow confused the two bugs, the new crash and repro didn't get reported to the mailing list.