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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, Tariq Toukan <tariqt@mellanox.com>,
        Willem de Bruijn <willemb@google.com>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: [PATCH 4.4 18/24] packet: fix reserve calculation
Date:   Tue, 12 Jun 2018 18:52:02 +0200
Message-Id: <20180612164817.509369173@linuxfoundation.org>
In-Reply-To: <20180612164816.587001852@linuxfoundation.org>
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4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Willem de Bruijn <willemb@google.com>

[ Upstream commit 9aad13b087ab0a588cd68259de618f100053360e ]

Commit b84bbaf7a6c8 ("packet: in packet_snd start writing at link
layer allocation") ensures that packet_snd always starts writing
the link layer header in reserved headroom allocated for this
purpose.

This is needed because packets may be shorter than hard_header_len,
in which case the space up to hard_header_len may be zeroed. But
that necessary padding is not accounted for in skb->len.

The fix, however, is buggy. It calls skb_push, which grows skb->len
when moving skb->data back. But in this case packet length should not
change.

Instead, call skb_reserve, which moves both skb->data and skb->tail
back, without changing length.

Fixes: b84bbaf7a6c8 ("packet: in packet_snd start writing at link layer allocation")
Reported-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/packet/af_packet.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2779,7 +2779,7 @@ static int packet_snd(struct socket *soc
 		if (unlikely(offset < 0))
 			goto out_free;
 	} else if (reserve) {
-		skb_push(skb, reserve);
+		skb_reserve(skb, -reserve);
 	}
 
 	/* Returns -EFAULT on error */