bpf: enforce recursion limit on redirects

Respect the stack's xmit_recursion limit for calls into dev_queue_xmit().
Currently, they are not handeled by the limiter when attached to clsact's
egress parent, for example, and a buggy program redirecting it to the
same device again could run into stack overflow eventually. It would be
good if we could notify an admin to give him a chance to react. We reuse
xmit_recursion instead of having one private to eBPF, so that the stack's
current recursion depth will be taken into account as well. Follow-up to
commit 3896d655f4 ("bpf: introduce bpf_clone_redirect() helper") and
27b29f6305 ("bpf: add bpf_redirect() helper").

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3144,8 +3144,6 @@ static void skb_update_prio(struct sk_buff *skb)
 DEFINE_PER_CPU(int, xmit_recursion);
 EXPORT_SYMBOL(xmit_recursion);
 
-#define RECURSION_LIMIT 10
-
 /**
  *	dev_loopback_xmit - loop back @skb
  *	@net: network namespace this loopback is happening in
@@ -3388,8 +3386,8 @@ static int __dev_queue_xmit(struct sk_buff *skb, void *accel_priv)
 		int cpu = smp_processor_id(); /* ok because BHs are off */
 
 		if (txq->xmit_lock_owner != cpu) {
-
-			if (__this_cpu_read(xmit_recursion) > RECURSION_LIMIT)
+			if (unlikely(__this_cpu_read(xmit_recursion) >
+				     XMIT_RECURSION_LIMIT))
 				goto recursion_alert;
 
 			skb = validate_xmit_skb(skb, dev);