linux-kernel - [RFC PATCH net v2] ipv4: ip_gre: Fix drops of small packets in ipgre

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Message-Id: <20240924013039.29200-1-littlesmilingcloud@gmail.com>
Date: Tue, 24 Sep 2024 04:30:40 +0300
From: Anton Danilov <littlesmilingcloud@...il.com>
To: netdev@...r.kernel.org
Cc: Anton Danilov <littlesmilingcloud@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Shigeru Yoshida <syoshida@...hat.com>,
	Suman Ghosh <sumang@...vell.com>,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH net v2] ipv4: ip_gre: Fix drops of small packets in ipgre_xmit

Regression Description:

Depending on the GRE tunnel device options, small packets are being
dropped. This occurs because the pskb_network_may_pull function fails due
to insufficient space in the network header. For example, if only the key
option is specified for the tunnel device, packets of sizes up to 27
(including the IPv4 header itself) will be dropped. This affects both
locally originated and forwarded packets in the DMVPN-like setups.

How to reproduce (for local originated packets):

  ip link add dev gre1 type gre ikey 1.9.8.4 okey 1.9.8.4 \
          local <your-ip> remote 0.0.0.0

  ip link set mtu 1400 dev gre1
  ip link set up dev gre1
  ip address add 192.168.13.1/24 dev gre1
  ip neighbor add 192.168.13.2 lladdr <remote-ip> dev gre1
  ping -s 1374 -c 10 192.168.13.2
  tcpdump -vni gre1
  tcpdump -vni <your-ext-iface> 'ip proto 47'
  ip -s -s -d link show dev gre1

Solution:

Use the pskb_may_pull function instead the pskb_network_may_pull.

Fixes: 80d875cfc9d3 ("ipv4: ip_gre: Avoid skb_pull() failure in ipgre_xmit()")
Signed-off-by: Anton Danilov <littlesmilingcloud@...il.com>
---
v1 -> v2 :
- Fix the reproduce commands
- Move out the 'tnl_params' assignment line to the more suitable place
with Eric's suggestion
https://lore.kernel.org/netdev/CANn89iJoMcxe6xAOE=QGfqmOa1p+_ssSr_2y4KUJr-Qap3xk0Q@mail.gmail.com/
---
 net/ipv4/ip_gre.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 5f6fd382af38..f1f31ebfc793 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -662,11 +662,11 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
 		if (skb_cow_head(skb, 0))
 			goto free_skb;
 
-		tnl_params = (const struct iphdr *)skb->data;
-
-		if (!pskb_network_may_pull(skb, pull_len))
+		if (!pskb_may_pull(skb, pull_len))
 			goto free_skb;
 
+		tnl_params = (const struct iphdr *)skb->data;
+
 		/* ip_tunnel_xmit() needs skb->data pointing to gre header. */
 		skb_pull(skb, pull_len);
 		skb_reset_mac_header(skb);
-- 
2.39.2