ipv4: ip_gre: Avoid skb_pull() failure in ipgre_xmit()

In ipgre_xmit(), skb_pull() may fail even if pskb_inet_may_pull() returns
true. For example, applications can use PF_PACKET to create a malformed
packet with no IP header. This type of packet causes a problem such as
uninit-value access.

This patch ensures that skb_pull() can pull the required size by checking
the skb with pskb_network_may_pull() before skb_pull().

Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
Signed-off-by: Shigeru Yoshida <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Reviewed-by: Suman Ghosh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/ipv4/ip_gre.c

···
 	}
 
 	if (dev->header_ops) {
+		int pull_len = tunnel->hlen + sizeof(struct iphdr);
+
 		if (skb_cow_head(skb, 0))
 			goto free_skb;
 
 		tnl_params = (const struct iphdr *)skb->data;
 
-		/* Pull skb since ip_tunnel_xmit() needs skb->data pointing
-		 * to gre header.
-		 */
-		skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
+		if (!pskb_network_may_pull(skb, pull_len))
+			goto free_skb;
+
+		/* ip_tunnel_xmit() needs skb->data pointing to gre header. */
+		skb_pull(skb, pull_len);
 		skb_reset_mac_header(skb);
 
 		if (skb->ip_summed == CHECKSUM_PARTIAL &&