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Subject[PATCH 4.4 20/38] net: lapb: Copy the skb before sending a packet
From: Xie He <>

[ Upstream commit 88c7a9fd9bdd3e453f04018920964c6f848a591a ]

When sending a packet, we will prepend it with an LAPB header.
This modifies the shared parts of a cloned skb, so we should copy the
skb rather than just clone it, before we prepend the header.

In "Documentation/networking/driver.rst" (the 2nd point), it states
that drivers shouldn't modify the shared parts of a cloned skb when

The "dev_queue_xmit_nit" function in "net/core/dev.c", which is called
when an skb is being sent, clones the skb and sents the clone to
AF_PACKET sockets. Because the LAPB drivers first remove a 1-byte
pseudo-header before handing over the skb to us, if we don't copy the
skb before prepending the LAPB header, the first byte of the packets
received on AF_PACKET sockets can be corrupted.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xie He <>
Acked-by: Martin Schiller <>
Signed-off-by: Jakub Kicinski <>
Signed-off-by: Sasha Levin <>
net/lapb/lapb_out.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/lapb/lapb_out.c b/net/lapb/lapb_out.c
index ba4d015bd1a67..7cbb77b7479a6 100644
--- a/net/lapb/lapb_out.c
+++ b/net/lapb/lapb_out.c
@@ -87,7 +87,8 @@ void lapb_kick(struct lapb_cb *lapb)
skb = skb_dequeue(&lapb->write_queue);

do {
- if ((skbn = skb_clone(skb, GFP_ATOMIC)) == NULL) {
+ skbn = skb_copy(skb, GFP_ATOMIC);
+ if (!skbn) {
skb_queue_head(&lapb->write_queue, skb);

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