bpf: Check skb->transport_header is set in bpf_skb_check_mtu

The bpf_skb_check_mtu helper needs to use skb->transport_header when
the BPF_MTU_CHK_SEGS flag is used:

	bpf_skb_check_mtu(skb, ifindex, &mtu_len, 0, BPF_MTU_CHK_SEGS)

The transport_header is not always set. There is a WARN_ON_ONCE
report when CONFIG_DEBUG_NET is enabled + skb->gso_size is set +
bpf_prog_test_run is used:

WARNING: CPU: 1 PID: 2216 at ./include/linux/skbuff.h:3071
 skb_gso_validate_network_len
 bpf_skb_check_mtu
 bpf_prog_3920e25740a41171_tc_chk_segs_flag # A test in the next patch
 bpf_test_run
 bpf_prog_test_run_skb

For a normal ingress skb (not test_run), skb_reset_transport_header
is performed but there is plan to avoid setting it as described in
commit 2170a1f ("net: no longer reset transport_header in __netif_receive_skb_core()").

This patch fixes the bpf helper by checking
skb_transport_header_was_set(). The check is done just before
skb->transport_header is used, to avoid breaking the existing bpf prog.
The WARN_ON_ONCE is limited to bpf_prog_test_run, so targeting bpf-next.

Fixes: 34b2021 ("bpf: Add BPF-helper for MTU checking")
Cc: Jesper Dangaard Brouer <[email protected]>
Reported-by: Kaiyan Mei <[email protected]>
Reported-by: Yinhao Hu <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Martin KaFai Lau

net/core/filter.c

@@ -6429,9 +6429,12 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 	 */
 	if (skb_is_gso(skb)) {
 		ret = BPF_MTU_CHK_RET_SUCCESS;
-		if (flags & BPF_MTU_CHK_SEGS &&
-		    !skb_gso_validate_network_len(skb, mtu))
-			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
+		if (flags & BPF_MTU_CHK_SEGS) {
+			if (!skb_transport_header_was_set(skb))
+				return -EINVAL;
+			if (!skb_gso_validate_network_len(skb, mtu))
+				ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
+		}
 	}
 out:
 	*mtu_len = mtu;