[PATCH 4.19 132/168] drivers/net/wan/lapbether: Added needed_headroom and a skb->len check

Greg Kroah-Hartman
2020-08-17T17:39:41Z

From: Xie He <xie.he.0141@gmail.com>

[ Upstream commit c7ca03c216acb14466a713fedf1b9f2c24994ef2 ]

1. Added a skb->len check

This driver expects upper layers to include a pseudo header of 1 byte
when passing down a skb for transmission. This driver will read this
1-byte header. This patch added a skb->len check before reading the
header to make sure the header exists.

2. Changed to use needed_headroom instead of hard_header_len to request
necessary headroom to be allocated

In net/packet/af_packet.c, the function packet_snd first reserves a
headroom of length (dev->hard_header_len + dev->needed_headroom).
Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
which calls dev->header_ops->create, to create the link layer header.
If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
length (dev->hard_header_len), and assumes the user to provide the
appropriate link layer header.

So according to the logic of af_packet.c, dev->hard_header_len should
be the length of the header that would be created by
dev->header_ops->create.

However, this driver doesn't provide dev->header_ops, so logically
dev->hard_header_len should be 0.

So we should use dev->needed_headroom instead of dev->hard_header_len
to request necessary headroom to be allocated.

This change fixes kernel panic when this driver is used with AF_PACKET
SOCK_RAW sockets.

Call stack when panic:

[ 168.399197] skbuff: skb_under_panic: text:ffffffff819d95fb len:20
put:14 head:ffff8882704c0a00 data:ffff8882704c09fd tail:0x11 end:0xc0
dev:veth0
...
[ 168.399255] Call Trace:
[ 168.399259] skb_push.cold+0x14/0x24
[ 168.399262] eth_header+0x2b/0xc0
[ 168.399267] lapbeth_data_transmit+0x9a/0xb0 [lapbether]
[ 168.399275] lapb_data_transmit+0x22/0x2c [lapb]
[ 168.399277] lapb_transmit_buffer+0x71/0xb0 [lapb]
[ 168.399279] lapb_kick+0xe3/0x1c0 [lapb]
[ 168.399281] lapb_data_request+0x76/0xc0 [lapb]
[ 168.399283] lapbeth_xmit+0x56/0x90 [lapbether]
[ 168.399286] dev_hard_start_xmit+0x91/0x1f0
[ 168.399289] ? irq_init_percpu_irqstack+0xc0/0x100
[ 168.399291] __dev_queue_xmit+0x721/0x8e0
[ 168.399295] ? packet_parse_headers.isra.0+0xd2/0x110
[ 168.399297] dev_queue_xmit+0x10/0x20
[ 168.399298] packet_sendmsg+0xbf0/0x19b0
......

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Cc: Brian Norris <briannorris@chromium.org>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/wan/lapbether.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -160,6 +160,12 @@ static netdev_tx_t lapbeth_xmit(struct s
if (!netif_running(dev))
goto drop;

+ /* There should be a pseudo header of 1 byte added by upper layers.
+ * Check to make sure it is there before reading it.
+ */
+ if (skb->len < 1)
+ goto drop;
+
switch (skb->data[0]) {
case X25_IFACE_DATA:
break;
@@ -308,6 +314,7 @@ static void lapbeth_setup(struct net_dev
dev->netdev_ops = &lapbeth_netdev_ops;
dev->needs_free_netdev = true;
dev->type = ARPHRD_X25;
+ dev->hard_header_len = 0;
dev->mtu = 1000;
dev->addr_len = 0;
}
@@ -334,7 +341,8 @@ static int lapbeth_new_device(struct net
* then this driver prepends a length field of 2 bytes,
* then the underlying Ethernet device prepends its own header.
*/
- ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
+ ndev->needed_headroom = -1 + 3 + 2 + dev->hard_header_len
+ + dev->needed_headroom;

lapbeth = netdev_priv(ndev);
lapbeth->axdev = ndev;