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From:   Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
To:     davem@davemloft.net, yoshfuji@linux-ipv6.org, dsahern@kernel.org,
        kuba@kernel.org, willemdebruijn.kernel@gmail.com
Cc:     Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzbot+ff8e1b9f2f36481e2efc@syzkaller.appspotmail.com
Subject: [PATCH] ip_gre/ip6_gre: add check for invalid csum_start
Date:   Thu, 19 Aug 2021 20:04:47 +0530
Message-Id: <20210819143447.314539-1-chouhan.shreyansh630@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

If we get a ip gre packet with TUNNEL_CSUM set, an invalid csum_start
value causes skb->csum_start offset to be less than the offset for
skb->data after we pull the ip header from the packet during the
ipgre_xmit call.

This patch adds a sanity check to gre_handle_offloads, which checks the
validity of skb->csum_start after we have pulled the ip header from the
packet in the ipgre_xmit call.

Reported-by: syzbot+ff8e1b9f2f36481e2efc@syzkaller.appspotmail.com
Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
---
 net/ipv4/ip_gre.c  | 2 ++
 net/ipv6/ip6_gre.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 12dca0c85f3c..95419b7adf5c 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -473,6 +473,8 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
 
 static int gre_handle_offloads(struct sk_buff *skb, bool csum)
 {
+	if (csum && skb_checksum_start(skb) < skb->data)
+		return -EINVAL;
 	return iptunnel_handle_offloads(skb, csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
 }
 
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index bc224f917bbd..7a5e90e09363 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -629,6 +629,8 @@ static int gre_rcv(struct sk_buff *skb)
 
 static int gre_handle_offloads(struct sk_buff *skb, bool csum)
 {
+	if (csum && skb_checksum_start(skb) < skb->data)
+		return -EINVAL;
 	return iptunnel_handle_offloads(skb,
 					csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
 }
-- 
2.31.1