On Tue, Apr 07, 2026 at 09:25:06PM -0700, Kuniyuki Iwashima wrote: > > > sashiko has flagged a similar issue with larger scope. > > > Please take a look. Thanks. > > > > > > https://sashiko.dev/#/patchset/20260403015851.148209-1-jiayuan.chen%40linux.dev > > > > > > Thanks a lot Martin, sashiko actually dug into a deeper issue here. > > > > Eric and Kuniyuki, > > > > I think the AI review has a point. Since BPF can modify skb fields, the > > following sequence still bypasses the protocol check in > > bpf_sk_assign_tcp_reqsk(): > > > > // for a UDP skb > > iph->protocol = TCP > > bpf_sk_assign_tcp_reqsk() > > iph->protocol = UDP > > > > On top of that, bpf_sk_assign() already has the same problem — it doesn't > > validate L4 protocol at all. > > Sigh... honestly it does not make sense to me to add changes > in the common fast path to protect someone with bpf capability > shooting oneself in the foot. > > On top of L4 validation in bpf_sk_assign() and bpf_sk_assign_tcp_reqsk(), > can't we mark such an skb immutable after the helpers and catch > subsequent writes to skb->data on the verifier ?
Clearing the skb->sk in a helper like bpf_skb_store_bytes or rejecting direct writes to skb->data could break existing bpf program. I suspect adding a simple iph->protocol/ip6h->nexthdr check to the helper (e.g. bpf_sk_assign) could also break some tunneling use cases (e.g. ipip) also. > > > > > > So I think we should add a check matching skb against sk in > > skb_steal_sock() instead of adding check in bpf helper. Maybe limit the check to the '*prefetched' case in skb_steal_sock(). FWIW, in the early days of bpf_sk_assign, a tc bpf program could only get hold of a tcp_sock. Later, bpf_map_lookup_elem(&sock_map) was allowed in tc, and then udp/unix sock support was also added to sock_map. There have been discussions on tc bpf programs being able to do bpf_map_lookup_elem(&sock_map) to get a unix_sock. AFAIK, this looked-up unix_sock can be used in bpf_sk_assign. It probably makes sense for bpf_sk_assign to reject all non-tcp/non-udp sk.
