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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Xie He <xie.he.0141@gmail.com>,
        Martin Schiller <ms@dev.tdt.de>,
        Jakub Kicinski <kuba@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.4 17/65] net: lapb: Copy the skb before sending a packet
Date:   Mon,  8 Feb 2021 16:00:49 +0100
Message-Id: <20210208145810.907176671@linuxfoundation.org>
In-Reply-To: <20210208145810.230485165@linuxfoundation.org>
References: <20210208145810.230485165@linuxfoundation.org>
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From: Xie He <xie.he.0141@gmail.com>

[ 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
transmitting.

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 <xie.he.0141@gmail.com>
Acked-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20210201055706.415842-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 7a4d0715d1c32..a966d29c772d9 100644
--- a/net/lapb/lapb_out.c
+++ b/net/lapb/lapb_out.c
@@ -82,7 +82,8 @@ 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);
 				break;
 			}
-- 
2.27.0