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Date:   Mon, 19 Oct 2020 03:49:42 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Krzysztof Halasa <khc@...waw.pl>
Cc:     Xie He <xie.he.0141@...il.com>
Subject: [PATCH net] drivers/net/wan/hdlc: In hdlc_rcv, check to make sure dev is an HDLC device

The hdlc_rcv function is used as hdlc_packet_type.func to process any
skb received in the kernel with skb->protocol == htons(ETH_P_HDLC).
The purpose of this function is to provide second-stage processing for
skbs not assigned a "real" L3 skb->protocol value in the first stage.

This function assumes the device from which the skb is received is an
HDLC device (a device created by this module). It assumes that
netdev_priv(dev) returns a pointer to "struct hdlc_device".

However, it is possible that some driver in the kernel (not necessarily
in our control) submits a received skb with skb->protocol ==
htons(ETH_P_HDLC), from a non-HDLC device. In this case, the skb would
still be received by hdlc_rcv. This will cause problems.

hdlc_rcv should be able to recognize and drop invalid skbs. It should
first make sure "dev" is actually an HDLC device, before starting its
processing.

To reliably check if a device is an HDLC device, we can check if its
dev->netdev_ops->ndo_start_xmit == hdlc_start_xmit, because all HDLC
devices are required to set their ndo_start_xmit to hdlc_start_xmit
(and all non-HDLC devices would not set their ndo_start_xmit to this).

Cc: Krzysztof Halasa <khc@...waw.pl>
Signed-off-by: Xie He <xie.he.0141@...il.com>
---
 drivers/net/wan/hdlc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/hdlc.c b/drivers/net/wan/hdlc.c
index 9b00708676cf..0a392fb9aff8 100644
--- a/drivers/net/wan/hdlc.c
+++ b/drivers/net/wan/hdlc.c
@@ -46,7 +46,15 @@ static struct hdlc_proto *first_proto;
 static int hdlc_rcv(struct sk_buff *skb, struct net_device *dev,
 		    struct packet_type *p, struct net_device *orig_dev)
 {
-	struct hdlc_device *hdlc = dev_to_hdlc(dev);
+	struct hdlc_device *hdlc;
+
+	/* First make sure "dev" is an HDLC device */
+	if (dev->netdev_ops->ndo_start_xmit != hdlc_start_xmit) {
+		kfree_skb(skb);
+		return NET_RX_SUCCESS;
+	}
+
+	hdlc = dev_to_hdlc(dev);
 
 	if (!net_eq(dev_net(dev), &init_net)) {
 		kfree_skb(skb);
-- 
2.25.1

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