ip_gre: test csum_start instead of transport header

GRE with TUNNEL_CSUM will apply local checksum offload on
CHECKSUM_PARTIAL packets.

ipgre_xmit must validate csum_start after an optional skb_pull,
else lco_csum may trigger an overflow. The original check was

	if (csum && skb_checksum_start(skb) < skb->data)
		return -EINVAL;

This had false positives when skb_checksum_start is undefined:
when ip_summed is not CHECKSUM_PARTIAL. A discussed refinement
was straightforward

	if (csum && skb->ip_summed == CHECKSUM_PARTIAL &&
	    skb_checksum_start(skb) < skb->data)
		return -EINVAL;

But was eventually revised more thoroughly:
- restrict the check to the only branch where needed, in an
  uncommon GRE path that uses header_ops and calls skb_pull.
- test skb_transport_header, which is set along with csum_start
  in skb_partial_csum_set in the normal header_ops datapath.

Turns out skbs can arrive in this branch without the transport
header set, e.g., through BPF redirection.

Revise the check back to check csum_start directly, and only if
CHECKSUM_PARTIAL. Do leave the check in the updated location.
Check field regardless of whether TUNNEL_CSUM is configured.

Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/
Link: https://lore.kernel.org/all/20210902193447.94039-2-willemdebruijn.kernel@gmail.com/T/#u
Fixes: 8a0ed250f911 ("ip_gre: validate csum_start only on pull")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20220606132107.3582565-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 7e474a85deaf560d49230791dca1d0d8319365ed..3b9cd487075af20fa31ac4786a6f124ff080270e 100644 (file)
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -629,21 +629,20 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
        }
 
        if (dev->header_ops) {
-               const int pull_len = tunnel->hlen + sizeof(struct iphdr);
-
                if (skb_cow_head(skb, 0))
                        goto free_skb;
 
                tnl_params = (const struct iphdr *)skb->data;
 
-               if (pull_len > skb_transport_offset(skb))
-                       goto free_skb;
-
                /* Pull skb since ip_tunnel_xmit() needs skb->data pointing
                 * to gre header.
                 */
-               skb_pull(skb, pull_len);
+               skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
                skb_reset_mac_header(skb);
+
+               if (skb->ip_summed == CHECKSUM_PARTIAL &&
+                   skb_checksum_start(skb) < skb->data)
+                       goto free_skb;
        } else {
                if (skb_cow_head(skb, dev->needed_headroom))
                        goto free_skb;