net: skbuff: ensure LSE is pullable before decrementing the MPLS ttl

skb_mpls_dec_ttl() reads the LSE without ensuring that it is contained in
the skb "linear" area. Fix this calling pskb_may_pull() before reading the
current ttl.

Found by code inspection.

Fixes: 2a2ea508 ("net: sched: add mpls manipulation actions to TC")
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/53659f28be8bc336c113b5254dc637cc76bbae91.1606987074.git.dcaratti@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -5786,6 +5786,9 @@ int skb_mpls_dec_ttl(struct sk_buff *skb)
 	if (unlikely(!eth_p_mpls(skb->protocol)))
 		return -EINVAL;
 
+	if (!pskb_may_pull(skb, skb_network_offset(skb) + MPLS_HLEN))
+		return -ENOMEM;
+
 	lse = be32_to_cpu(mpls_hdr(skb)->label_stack_entry);
 	ttl = (lse & MPLS_LS_TTL_MASK) >> MPLS_LS_TTL_SHIFT;
 	if (!--ttl)