[PATCH 5.4 362/411] ip_gre: test csum_start instead of transport header

Greg Kroah-Hartman
2022-06-13T11:48:00Z

From: Willem de Bruijn <willemb@google.com>

[ Upstream commit 8d21e9963bec1aad2280cdd034c8993033ef2948 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/ip_gre.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 5b38d03f6d79..614410a6db44 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -602,21 +602,20 @@ 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;
--
2.35.1