bpf, sockmap: Avoid failures from skb_to_sgvec when skb has frag_list

When skb has a frag_list its possible for skb_to_sgvec() to fail. This
happens when the scatterlist has fewer elements to store pages than would
be needed for the initial skb plus any of its frags.

This case appears rare, but is possible when running an RX parser/verdict
programs exposed to the internet. Currently, when this happens we throw
an error, break the pipe, and kfree the msg. This effectively breaks the
application or forces it to do a retry.

Lets catch this case and handle it by doing an skb_linearize() on any
skb we receive with frags. At this point skb_to_sgvec should not fail
because the failing conditions would require frags to be in place.

Fixes: 604326b4 ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/160556576837.73229.14800682790808797635.stgit@john-XPS-13-9370

net/core/skmsg.c

@@ -423,9 +423,16 @@ static int sk_psock_skb_ingress_enqueue(struct sk_buff *skb,
 					struct sock *sk,
 					struct sk_msg *msg)
 {
-	int num_sge = skb_to_sgvec(skb, msg->sg.data, 0, skb->len);
-	int copied;
+	int num_sge, copied;
 
+	/* skb linearize may fail with ENOMEM, but lets simply try again
+	 * later if this happens. Under memory pressure we don't want to
+	 * drop the skb. We need to linearize the skb so that the mapping
+	 * in skb_to_sgvec can not error.
+	 */
+	if (skb_linearize(skb))
+		return -EAGAIN;
+	num_sge = skb_to_sgvec(skb, msg->sg.data, 0, skb->len);
 	if (unlikely(num_sge < 0)) {
 		kfree(msg);
 		return num_sge;