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wexu at redhat.com

2017-Nov-29 14:23 UTC

[PATCH net,stable v2] vhost: fix skb leak in handle_rx()

From: Wei Xu <wexu at redhat.com>

Matthew found a roughly 40% tcp throughput regression with commit
c67df11f(vhost_net: try batch dequing from skb array) as discussed
in the following thread:
https://www.mail-archive.com/netdev at vger.kernel.org/msg187936.html

Eventually we figured out that it was a skb leak in handle_rx()
when sending packets to the VM. This usually happens when a guest
can not drain out vq as fast as vhost fills in, afterwards it sets
off the traffic jam and leaks skb(s) which occurs as no headcount
to send on the vq from vhost side.

This can be avoided by making sure we have got enough headcount
before actually consuming a skb from the batched rx array while
transmitting, which is simply done by moving checking the zero
headcount a bit ahead.

Also strengthen the small possibility of leak in case of recvmsg()
fails by freeing the skb.

Signed-off-by: Wei Xu <wexu at redhat.com>
Reported-by: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
---
 drivers/vhost/net.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

v2:
- add Matthew as the reporter, thanks matthew.
- moving zero headcount check ahead instead of defer consuming skb
  due to jason and mst's comment.
- add freeing skb in favor of recvmsg() fails.

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8d626d7..e302e08 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
 		/* On error, stop handling until the next kick. */
 		if (unlikely(headcount < 0))
 			goto out;
-		if (nvq->rx_array)
-			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
-		/* On overrun, truncate and discard */
-		if (unlikely(headcount > UIO_MAXIOV)) {
-			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
-			err = sock->ops->recvmsg(sock, &msg,
-						 1, MSG_DONTWAIT | MSG_TRUNC);
-			pr_debug("Discarded rx packet: len %zd\n", sock_len);
-			continue;
-		}
 		/* OK, now we need to know about added descriptors. */
 		if (!headcount) {
 			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
@@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
 			 * they refilled. */
 			goto out;
 		}
+		if (nvq->rx_array)
+			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
+		/* On overrun, truncate and discard */
+		if (unlikely(headcount > UIO_MAXIOV)) {
+			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
+			err = sock->ops->recvmsg(sock, &msg,
+						 1, MSG_DONTWAIT | MSG_TRUNC);
+			if (unlikely(err != 1))
+				kfree_skb((struct sk_buff *)msg.msg_control);
+			pr_debug("Discarded rx packet: len %zd\n", sock_len);
+			continue;
+		}
 		/* We don't need to be notified again. */
 		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
 		fixup = msg.msg_iter;
@@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
 			pr_debug("Discarded rx packet: "
 				 " len %d, expected %zd\n", err, sock_len);
 			vhost_discard_vq_desc(vq, headcount);
+			kfree_skb((struct sk_buff *)msg.msg_control);
 			continue;
 		}
 		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
-- 
1.8.3.1

Jason Wang

2017-Nov-29 14:43 UTC

head link

[PATCH net,stable v2] vhost: fix skb leak in handle_rx()

On 2017?11?29? 22:23, wexu at redhat.com wrote:> From: Wei Xu <wexu at redhat.com>
>
> Matthew found a roughly 40% tcp throughput regression with commit
> c67df11f(vhost_net: try batch dequing from skb array) as discussed
> in the following thread:
> https://www.mail-archive.com/netdev at vger.kernel.org/msg187936.html
>
> Eventually we figured out that it was a skb leak in handle_rx()
> when sending packets to the VM. This usually happens when a guest
> can not drain out vq as fast as vhost fills in, afterwards it sets
> off the traffic jam and leaks skb(s) which occurs as no headcount
> to send on the vq from vhost side.
>
> This can be avoided by making sure we have got enough headcount
> before actually consuming a skb from the batched rx array while
> transmitting, which is simply done by moving checking the zero
> headcount a bit ahead.
>
> Also strengthen the small possibility of leak in case of recvmsg()
> fails by freeing the skb.
>
> Signed-off-by: Wei Xu <wexu at redhat.com>
> Reported-by: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
> ---
>   drivers/vhost/net.c | 23 +++++++++++++----------
>   1 file changed, 13 insertions(+), 10 deletions(-)
>
> v2:
> - add Matthew as the reporter, thanks matthew.
> - moving zero headcount check ahead instead of defer consuming skb
>    due to jason and mst's comment.
> - add freeing skb in favor of recvmsg() fails.
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e302e08 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
>   		/* On error, stop handling until the next kick. */
>   		if (unlikely(headcount < 0))
>   			goto out;
> -		if (nvq->rx_array)
> -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> -		/* On overrun, truncate and discard */
> -		if (unlikely(headcount > UIO_MAXIOV)) {
> -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> -			err = sock->ops->recvmsg(sock, &msg,
> -						 1, MSG_DONTWAIT | MSG_TRUNC);
> -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> -			continue;
> -		}
>   		/* OK, now we need to know about added descriptors. */
>   		if (!headcount) {
>   			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> @@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
>   			 * they refilled. */
>   			goto out;
>   		}
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> +		/* On overrun, truncate and discard */
> +		if (unlikely(headcount > UIO_MAXIOV)) {
> +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> +			err = sock->ops->recvmsg(sock, &msg,
> +						 1, MSG_DONTWAIT | MSG_TRUNC);
> +			if (unlikely(err != 1))
> +				kfree_skb((struct sk_buff *)msg.msg_control);
I think we'd better fix this in tun/tap (better in another patch) 
otherwise it lead to an odd API: some case skb were freed in recvmsg() 
but caller still need to deal with the rest case.

Thanks
> +			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> +			continue;
> +		}
>   		/* We don't need to be notified again. */
>   		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
>   		fixup = msg.msg_iter;
> @@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
>   			pr_debug("Discarded rx packet: "
>   				 " len %d, expected %zd\n", err, sock_len);
>   			vhost_discard_vq_desc(vq, headcount);
> +			kfree_skb((struct sk_buff *)msg.msg_control);
>   			continue;
>   		}
>   		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */

Michael S. Tsirkin

2017-Nov-29 15:31 UTC

head link

[PATCH net,stable v2] vhost: fix skb leak in handle_rx()

On Wed, Nov 29, 2017 at 09:23:24AM -0500, wexu at redhat.com
wrote:> From: Wei Xu <wexu at redhat.com>
> 
> Matthew found a roughly 40% tcp throughput regression with commit
> c67df11f(vhost_net: try batch dequing from skb array) as discussed
> in the following thread:
> https://www.mail-archive.com/netdev at vger.kernel.org/msg187936.html
> 
> Eventually we figured out that it was a skb leak in handle_rx()
> when sending packets to the VM. This usually happens when a guest
> can not drain out vq as fast as vhost fills in, afterwards it sets
> off the traffic jam and leaks skb(s) which occurs as no headcount
> to send on the vq from vhost side.
> 
> This can be avoided by making sure we have got enough headcount
> before actually consuming a skb from the batched rx array while
> transmitting, which is simply done by moving checking the zero
> headcount a bit ahead.
> 
> Also strengthen the small possibility of leak in case of recvmsg()
> fails by freeing the skb.
> 
> Signed-off-by: Wei Xu <wexu at redhat.com>
> Reported-by: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
> ---
>  drivers/vhost/net.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> v2:
> - add Matthew as the reporter, thanks matthew.
> - moving zero headcount check ahead instead of defer consuming skb
>   due to jason and mst's comment.
> - add freeing skb in favor of recvmsg() fails.
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e302e08 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
>  			goto out;
> -		if (nvq->rx_array)
> -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> -		/* On overrun, truncate and discard */
> -		if (unlikely(headcount > UIO_MAXIOV)) {
> -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> -			err = sock->ops->recvmsg(sock, &msg,
> -						 1, MSG_DONTWAIT | MSG_TRUNC);
> -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> -			continue;
> -		}
>  		/* OK, now we need to know about added descriptors. */
>  		if (!headcount) {
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> @@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
>  			 * they refilled. */
>  			goto out;
>  		}
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> +		/* On overrun, truncate and discard */
> +		if (unlikely(headcount > UIO_MAXIOV)) {
> +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> +			err = sock->ops->recvmsg(sock, &msg,
> +						 1, MSG_DONTWAIT | MSG_TRUNC);
> +			if (unlikely(err != 1))
Why 1? How is receiving 1 byte special or even possible?
Also, I wouldn't put an unlikely here. It's all error handling code
anyway.
> +				kfree_skb((struct sk_buff *)msg.msg_control);
You do not need a cast here.
Also, is it really safe to refer to msg_control here?
I'd rather keep a copy of the skb pointer and use it than assume
caller did not change it. But also see below.
> +			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> +			continue;
> +		}
>  		/* We don't need to be notified again. */
>  		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
>  		fixup = msg.msg_iter;
> @@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
>  			pr_debug("Discarded rx packet: "
>  				 " len %d, expected %zd\n", err, sock_len);
>  			vhost_discard_vq_desc(vq, headcount);
> +			kfree_skb((struct sk_buff *)msg.msg_control);
You do not need a cast here.

Also, we have

        ret = tun_put_user(tun, tfile, skb, to);
        if (unlikely(ret < 0))
                kfree_skb(skb);
        else
                consume_skb(skb);

        return ret;

So it looks like recvmsg actually always consumes the skb.
So I was wrong when I said you need to kfree it after
recv msg, and your original patch was good.

Jason, what do you think?
>  			continue;
>  		}
>  		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
> -- 
> 1.8.3.1

Jason Wang

2017-Nov-30 02:46 UTC

head link

[PATCH net,stable v2] vhost: fix skb leak in handle_rx()

On 2017?11?29? 23:31, Michael S. Tsirkin wrote:> On Wed, Nov 29, 2017 at 09:23:24AM -0500,wexu at redhat.com  wrote:
>> From: Wei Xu<wexu at redhat.com>
>>
>> Matthew found a roughly 40% tcp throughput regression with commit
>> c67df11f(vhost_net: try batch dequing from skb array) as discussed
>> in the following thread:
>> https://www.mail-archive.com/netdev at vger.kernel.org/msg187936.html
>>
>> Eventually we figured out that it was a skb leak in handle_rx()
>> when sending packets to the VM. This usually happens when a guest
>> can not drain out vq as fast as vhost fills in, afterwards it sets
>> off the traffic jam and leaks skb(s) which occurs as no headcount
>> to send on the vq from vhost side.
>>
>> This can be avoided by making sure we have got enough headcount
>> before actually consuming a skb from the batched rx array while
>> transmitting, which is simply done by moving checking the zero
>> headcount a bit ahead.
>>
>> Also strengthen the small possibility of leak in case of recvmsg()
>> fails by freeing the skb.
>>
>> Signed-off-by: Wei Xu<wexu at redhat.com>
>> Reported-by: Matthew Rosato<mjrosato at linux.vnet.ibm.com>
>> ---
>>   drivers/vhost/net.c | 23 +++++++++++++----------
>>   1 file changed, 13 insertions(+), 10 deletions(-)
>>
>> v2:
>> - add Matthew as the reporter, thanks matthew.
>> - moving zero headcount check ahead instead of defer consuming skb
>>    due to jason and mst's comment.
>> - add freeing skb in favor of recvmsg() fails.
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 8d626d7..e302e08 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
>>   		/* On error, stop handling until the next kick. */
>>   		if (unlikely(headcount < 0))
>>   			goto out;
>> -		if (nvq->rx_array)
>> -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
>> -		/* On overrun, truncate and discard */
>> -		if (unlikely(headcount > UIO_MAXIOV)) {
>> -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
>> -			err = sock->ops->recvmsg(sock, &msg,
>> -						 1, MSG_DONTWAIT | MSG_TRUNC);
>> -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
>> -			continue;
>> -		}
>>   		/* OK, now we need to know about added descriptors. */
>>   		if (!headcount) {
>>   			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>> @@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
>>   			 * they refilled. */
>>   			goto out;
>>   		}
>> +		if (nvq->rx_array)
>> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
>> +		/* On overrun, truncate and discard */
>> +		if (unlikely(headcount > UIO_MAXIOV)) {
>> +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
>> +			err = sock->ops->recvmsg(sock, &msg,
>> +						 1, MSG_DONTWAIT | MSG_TRUNC);
>> +			if (unlikely(err != 1))
> Why 1? How is receiving 1 byte special or even possible?
> Also, I wouldn't put an unlikely here. It's all error handling code
anyway.
>
>> +				kfree_skb((struct sk_buff *)msg.msg_control);
> You do not need a cast here.
> Also, is it really safe to refer to msg_control here?
> I'd rather keep a copy of the skb pointer and use it than assume
> caller did not change it. But also see below.
>
>> +			pr_debug("Discarded rx packet: len %zd\n", sock_len);
>> +			continue;
>> +		}
>>   		/* We don't need to be notified again. */
>>   		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
>>   		fixup = msg.msg_iter;
>> @@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
>>   			pr_debug("Discarded rx packet: "
>>   				 " len %d, expected %zd\n", err, sock_len);
>>   			vhost_discard_vq_desc(vq, headcount);
>> +			kfree_skb((struct sk_buff *)msg.msg_control);
> You do not need a cast here.
>
> Also, we have
>
>          ret = tun_put_user(tun, tfile, skb, to);
>          if (unlikely(ret < 0))
>                  kfree_skb(skb);
>          else
>                  consume_skb(skb);
>
>          return ret;
>
> So it looks like recvmsg actually always consumes the skb.
> So I was wrong when I said you need to kfree it after
> recv msg, and your original patch was good.
>
> Jason, what do you think?
>
tun_recvmsg() has the following check:

static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t 
total_len,
 ??? ??? ?????? int flags)
{
 ??? struct tun_file *tfile = container_of(sock, struct tun_file, socket);
 ??? struct tun_struct *tun = __tun_get(tfile);
 ??? int ret;

 ??? if (!tun)
 ??? ??? return -EBADFD;

 ??? if (flags & ~(MSG_DONTWAIT|MSG_TRUNC|MSG_ERRQUEUE)) {
 ??? ??? ret = -EINVAL;
 ??? ??? goto out;
 ??? }

And tun_do_read() has:

 ??? if (!iov_iter_count(to))
 ??? ??? return 0;

So I think we need free skb in those cases.

Thanks

Wei Xu

2017-Nov-30 04:45 UTC

head link

[PATCH net,stable v2] vhost: fix skb leak in handle_rx()

On Wed, Nov 29, 2017 at 10:43:33PM +0800, Jason Wang
wrote:> 
> 
> On 2017?11?29? 22:23, wexu at redhat.com wrote:
> > From: Wei Xu <wexu at redhat.com>
> > 
> > Matthew found a roughly 40% tcp throughput regression with commit
> > c67df11f(vhost_net: try batch dequing from skb array) as discussed
> > in the following thread:
> > https://www.mail-archive.com/netdev at vger.kernel.org/msg187936.html
> > 
> > Eventually we figured out that it was a skb leak in handle_rx()
> > when sending packets to the VM. This usually happens when a guest
> > can not drain out vq as fast as vhost fills in, afterwards it sets
> > off the traffic jam and leaks skb(s) which occurs as no headcount
> > to send on the vq from vhost side.
> > 
> > This can be avoided by making sure we have got enough headcount
> > before actually consuming a skb from the batched rx array while
> > transmitting, which is simply done by moving checking the zero
> > headcount a bit ahead.
> > 
> > Also strengthen the small possibility of leak in case of recvmsg()
> > fails by freeing the skb.
> > 
> > Signed-off-by: Wei Xu <wexu at redhat.com>
> > Reported-by: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
> > ---
> >   drivers/vhost/net.c | 23 +++++++++++++----------
> >   1 file changed, 13 insertions(+), 10 deletions(-)
> > 
> > v2:
> > - add Matthew as the reporter, thanks matthew.
> > - moving zero headcount check ahead instead of defer consuming skb
> >    due to jason and mst's comment.
> > - add freeing skb in favor of recvmsg() fails.
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 8d626d7..e302e08 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
> >   		/* On error, stop handling until the next kick. */
> >   		if (unlikely(headcount < 0))
> >   			goto out;
> > -		if (nvq->rx_array)
> > -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> > -		/* On overrun, truncate and discard */
> > -		if (unlikely(headcount > UIO_MAXIOV)) {
> > -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> > -			err = sock->ops->recvmsg(sock, &msg,
> > -						 1, MSG_DONTWAIT | MSG_TRUNC);
> > -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> > -			continue;
> > -		}
> >   		/* OK, now we need to know about added descriptors. */
> >   		if (!headcount) {
> >   			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> > @@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
> >   			 * they refilled. */
> >   			goto out;
> >   		}
> > +		if (nvq->rx_array)
> > +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> > +		/* On overrun, truncate and discard */
> > +		if (unlikely(headcount > UIO_MAXIOV)) {
> > +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> > +			err = sock->ops->recvmsg(sock, &msg,
> > +						 1, MSG_DONTWAIT | MSG_TRUNC);
> > +			if (unlikely(err != 1))
> > +				kfree_skb((struct sk_buff *)msg.msg_control);
> 
> I think we'd better fix this in tun/tap (better in another patch)
otherwise
> it lead to an odd API: some case skb were freed in recvmsg() but caller
> still need to deal with the rest case.
Right, it is better to handle it in recvmsg().

Wei

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