Virtualization - Nov 2021 - [PATCH 1/2] vhost/vsock: fix incorrect used length reported to the guest

This is a follow-up to Micheal's patch [1] and the discussion with Halil and
Jason [2].

I made two patches, one to fix the problem and one for cleanup. This should
simplify the backport of the fix because we've had the problem since
vhost-vsock was introduced (v4.8) and that part has been touched a bit
recently.

Thanks,
Stefano

[1] https://lore.kernel.org/virtualization/20211122105822.onarsa4sydzxqynu at
steredhat/T/#t
[2] https://lore.kernel.org/virtualization/20211027022107.14357-1-jasowang at
redhat.com/T/#t

Stefano Garzarella (2):
  vhost/vsock: fix incorrect used length reported to the guest
  vhost/vsock: cleanup removing `len` variable

 drivers/vhost/vsock.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

-- 
2.31.1

The "used length" reported by calling vhost_add_used() must be the
number of bytes written by the device (using "in" buffers).

In vhost_vsock_handle_tx_kick() the device only reads the guest
buffers (they are all "out" buffers), without writing anything,
so we must pass 0 as "used length" to comply virtio spec.

Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
Cc: stable at vger.kernel.org
Reported-by: Halil Pasic <pasic at linux.ibm.com>
Suggested-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 drivers/vhost/vsock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 938aefbc75ec..4e3b95af7ee4 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -554,7 +554,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
*work)
 			virtio_transport_free_pkt(pkt);
 
 		len += sizeof(pkt->hdr);
-		vhost_add_used(vq, head, len);
+		vhost_add_used(vq, head, 0);
 		total_len += len;
 		added = true;
 	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
-- 
2.31.1

We can increment `total_len` directly and remove `len` since it
is no longer used for vhost_add_used().

Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 drivers/vhost/vsock.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 4e3b95af7ee4..d6ca1c7ad513 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -511,8 +511,6 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
*work)
 
 	vhost_disable_notify(&vsock->dev, vq);
 	do {
-		u32 len;
-
 		if (!vhost_vsock_more_replies(vsock)) {
 			/* Stop tx until the device processes already
 			 * pending replies.  Leave tx virtqueue
@@ -540,7 +538,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
*work)
 			continue;
 		}
 
-		len = pkt->len;
+		total_len += sizeof(pkt->hdr) + pkt->len;
 
 		/* Deliver to monitoring devices all received packets */
 		virtio_transport_deliver_tap_pkt(pkt);
@@ -553,9 +551,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
*work)
 		else
 			virtio_transport_free_pkt(pkt);
 
-		len += sizeof(pkt->hdr);
 		vhost_add_used(vq, head, 0);
-		total_len += len;
 		added = true;
 	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
 
-- 
2.31.1

On Mon, Nov 22, 2021 at 05:35:23PM +0100, Stefano Garzarella
wrote:
> This is a follow-up to Micheal's patch [1] and the discussion with
Halil and
> Jason [2].
> 
> I made two patches, one to fix the problem and one for cleanup. This should
> simplify the backport of the fix because we've had the problem since
> vhost-vsock was introduced (v4.8) and that part has been touched a bit
> recently.
> 
> Thanks,
> Stefano
> 
> [1] https://lore.kernel.org/virtualization/20211122105822.onarsa4sydzxqynu
at steredhat/T/#t
> [2] https://lore.kernel.org/virtualization/20211027022107.14357-1-jasowang
at redhat.com/T/#t
> 
> Stefano Garzarella (2):
>   vhost/vsock: fix incorrect used length reported to the guest
>   vhost/vsock: cleanup removing `len` variable
> 
>  drivers/vhost/vsock.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> -- 
> 2.31.1
> 
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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