Early Access — Mondoo Vulnerability Intelligence is currently in preview.
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.