netdev - [PATCH] drivers/net/wan/lapbether.c: Fixed kernel panic when used with AF

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Message-Id: <20200528032134.13752-1-hexie3605@gmail.com>
Date:   Wed, 27 May 2020 20:21:33 -0700
From:   Xie He <hexie3605@...il.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        Xie He <hexie3605@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] drivers/net/wan/lapbether.c: Fixed kernel panic when used with AF_PACKET sockets

When we use "AF_PACKET" sockets to send data directly over LAPB over
Ethernet using this driver, the kernel will panic because of
insufficient header space allocated in the "sk_buff" struct.

The header space needs 18 bytes because:
  the lapbether driver will remove a pseudo header of 1 byte;
  the lapb module will prepend the LAPB header of 2 or 3 bytes;
  the lapbether driver will prepend a length field of 2 bytes and the
Ethernet header of 14 bytes.

So -1 + 3 + 16 = 18.

Signed-off-by: Xie He <hexie3605@...il.com>
---
 drivers/net/wan/lapbether.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index e30d91a38cfb..619413f5d432 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -303,7 +303,8 @@ static void lapbeth_setup(struct net_device *dev)
 	dev->netdev_ops	     = &lapbeth_netdev_ops;
 	dev->needs_free_netdev = true;
 	dev->type            = ARPHRD_X25;
-	dev->hard_header_len = 3;
+	/* 18 = -1 (lapbether) + 3 (lapb) + 16 (lapbether) */
+	dev->hard_header_len = 18;
 	dev->mtu             = 1000;
 	dev->addr_len        = 0;
 }
-- 
2.25.1