[PATCH][VHOST] fix race with guest on multi-buffer used buffer updates

[David L Stevens]

[for Michael Tsirkin's vhost development git tree]

This patch fixes a race between guest and host when
adding used buffers wraps the ring. Without it, guests
can see partial packets before num_buffers is set in
the vnet header.

Signed-off-by: David L Stevens <dlstev...@us.ibm.com>

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 7f2568d..74790ab 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1065,14 +1065,6 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
                vq_err(vq, "Failed to write used");
                return -EFAULT;
        }
-       /* Make sure buffer is written before we update index. */
-       smp_wmb();
-       if (put_user(vq->last_used_idx + count, &vq->used->idx)) {
-               vq_err(vq, "Failed to increment used idx");
-               return -EFAULT;
-       }
-       if (unlikely(vq->log_used))
-               vhost_log_used(vq, used);
        vq->last_used_idx += count;
        return 0;
 }
@@ -1093,7 +1085,17 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct 
vring_used_elem *heads,
                heads += n;
                count -= n;
        }
-       return __vhost_add_used_n(vq, heads, count);
+       r = __vhost_add_used_n(vq, heads, count);
+
+       /* Make sure buffer is written before we update index. */
+       smp_wmb();
+       if (put_user(vq->last_used_idx, &vq->used->idx)) {
+               vq_err(vq, "Failed to increment used idx");
+               return -EFAULT;
+       }
+       if (unlikely(vq->log_used))
+               vhost_log_used(vq, vq->used->ring + start);
+       return r;
 }
 
 /* This actually signals the guest, using eventfd. */


--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html