On Fri, Sep 25, 2020 at 09:24:02AM -0700, John Fastabend wrote:
[ ... ]
> Hi Martin,
>
> One piece I'm missing is how does this handle null pointer dereferences
> from network side when reading BTF objects? I've not got through all the
> code yet so maybe I'm just not there yet.
>
> For example,
>
> > diff --git a/tools/testing/selftests/bpf/bpf_tcp_helpers.h b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > index a0e8b3758bd7..2915664c335d 100644
> > --- a/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > +++ b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > @@ -16,6 +16,7 @@ BPF_PROG(name, args)
> >
> > struct sock_common {
> > unsigned char skc_state;
> > + __u16 skc_num;
> > } __attribute__((preserve_access_index));
> >
> > enum sk_pacing {
> > @@ -45,6 +46,10 @@ struct inet_connection_sock {
> > __u64 icsk_ca_priv[104 / sizeof(__u64)];
> > } __attribute__((preserve_access_index));
> >
> > +struct request_sock {
> > + struct sock_common __req_common;
> > +} __attribute__((preserve_access_index));
> > +
> > struct tcp_sock {
> > struct inet_connection_sock inet_conn;
>
> add some pointer from tcp_sock which is likely not set so should be NULL,
>
> struct tcp_fastopen_request *fastopen_req;
>
> [...]
>
> > + if (bpf_skc->state == BPF_TCP_NEW_SYN_RECV) {
> > + struct request_sock *req_sk;
> > +
> > + req_sk = (struct request_sock *)bpf_skc_to_tcp_request_sock(bpf_skc);
> > + if (!req_sk) {
> > + LOG();
> > + goto release;
> > + }
> > +
> > + if (bpf_sk_assign(skb, req_sk, 0)) {
> > + LOG();
> > + goto release;
> > + }
> > +
> > + req_sk_sport = req_sk->__req_common.skc_num;
> > +
> > + bpf_sk_release(req_sk);
> > + return TC_ACT_OK;
> > + } else if (bpf_skc->state == BPF_TCP_LISTEN) {
> > + struct tcp_sock *tp;
> > +
> > + tp = bpf_skc_to_tcp_sock(bpf_skc);
> > + if (!tp) {
> > + LOG();
> > + goto release;
> > + }
> > +
> > + if (bpf_sk_assign(skb, tp, 0)) {
> > + LOG();
> > + goto release;
> > + }
> > +
>
>
> Then use it here without a null check in the BPF program,
>
> fastopen = tp->fastopen_req;
fastopen is in PTR_TO_BTF_ID here.
> if (fastopen->size > 0x1234)
This load will be marked with BPF_PROBE_MEM.
> (do something)
>
> > + listen_tp_sport = tp->inet_conn.icsk_inet.sk.__sk_common.skc_num;
> > +
> > + test_syncookie_helper(ip6h, th, tp, skb);
> > + bpf_sk_release(tp);
> > + return TC_ACT_OK;
> > + }
>
> My quick check shows this didn't actually fault and the xlated
> looks like it did the read and dereference. Must be missing
> something? We shouldn't have fault_handler set for cls_ingress.
By xlated, do you mean the interpreter mode? The LDX_PROBE_MEM
is done by bpf_probe_read_kernel() in bpf/core.c.
I don't think the handling of PTR_TO_BTF_ID is depending on
prog->type.
>
> Perhaps a comment in the cover letter about this would be
> helpful? Or if I'm just being dense this morning let me know
> as well. ;)
>
