Subject: Re: [PATCH net-2.6 alt.3] af_packet: Don't use skb after dev_queue_xmit()

From: Jarek Poplawski <[email-masked]>
Date: Mon, 11 Jan 2010 08:04:19 +0000

On 10-01-2010 22:51, David Miller wrote:
> From: Jarek Poplawski <[email-masked]>
> Date: Sat, 9 Jan 2010 13:38:27 +0100
> 
>> tpacket_snd() can change and kfree an skb after dev_queue_xmit(),
>> which is illegal.
>>
>> With debugging by: Stephen Hemminger <[email-masked]>
>>
>> Reported-by: Michael Breuer <[email-masked]>
>> Tested-by: Michael Breuer <[email-masked]>
>> Signed-off-by: Jarek Poplawski <[email-masked]>
>> Acked-by: Stephen Hemminger <[email-masked]>
> 
> Jarek, if this code path triggers, it will deadlock the
> send ring with your changes.
> 
> We will now leave the ring packet status in the "SENDING" state.
> 
> That's not right.
> 
> Then, if the application calls send again, we will just return
> immediately since we only make progress if the head ring entry is in
> SEND_REQUEST state.
> 
> This is really bogus behavior.  When the qdisc or mid-layer
> drops the packet, we should at least mark the packet state
> properly (which is what the current code would does, sans
> the "reference SKB after dev_queue_xmit()" issue).  And
> advance the packet ring pointer.
> 
> This way the user:
> 
> 1) can see that the packet got dropped and couldn't be sent
> 
> 2) can call send again to try sending the rest of the ring
> 
> Fix the use after dev_queue_xmit() issue, but don't change other side
> effects which are important for correct AF_PACKET TX ring semantics.

As I wrote already, I don't think this patch is wrong. Alas, we can't
both fix this bug and retain exactly current behaviour, at least
without deeper changes. And I doubt it's worth it if we ignore negative
dev_queue_xmit() return (drops also) at the same time.

Btw, there was an alternative fix (positively) tested - more radical,
but IMHO safe and appropriate at least as a temporary solution for
-stable:
http://permalink.gmane.org/gmane.linux.kernel/934761

Anyway, here is another try, with even more of the current semantics.
If you think it's better, I hope Michael can test it (and send his
Tested-by).

Thanks,
Jarek P.
----------------> (alternative 3)

Subject: af_packet: Don't use skb after dev_queue_xmit()

tpacket_snd() can change and kfree an skb after dev_queue_xmit(),
which is illegal.

With debugging by: Stephen Hemminger <[email-masked]>

Reported-by: Michael Breuer <[email-masked]>
Signed-off-by: Jarek Poplawski <[email-masked]>

Cc: Stephen Hemminger <[email-masked]>
---

 net/packet/af_packet.c |   19 ++++++++++++++-----
 1 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index e0516a2..f126d18 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1021,8 +1021,20 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 
 		status = TP_STATUS_SEND_REQUEST;
 		err = dev_queue_xmit(skb);
-		if (unlikely(err > 0 && (err = net_xmit_errno(err)) != 0))
-			goto out_xmit;
+		if (unlikely(err > 0)) {
+			err = net_xmit_errno(err);
+			if (err && __packet_get_status(po, ph) ==
+				   TP_STATUS_AVAILABLE) {
+				/* skb was destructed already */
+				skb = NULL;
+				goto out_status;
+			}
+			/*
+			 * skb was dropped but not destructed yet;
+			 * let's treat it like congestion or err < 0
+			 */
+			err = 0;
+		}
 		packet_increment_head(&po->tx_ring);
 		len_sum += tp_len;
 	} while (likely((ph != NULL) ||
@@ -1033,9 +1045,6 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 	err = len_sum;
 	goto out_put;
 
-out_xmit:
-	skb->destructor = sock_wfree;
-	atomic_dec(&po->tx_ring.pending);
 out_status:
 	__packet_set_status(po, ph, status);
 	kfree_skb(skb);