Virtualization - Sep 2010 - [PATCH] vhost-net: fix range checking in mrg bufs case

In mergeable buffer case, we use headcount, log_num
and seg as indexes in same-size arrays, and
we know that headcount <= seg and
log_num equals either 0 or seg.

Therefore, the right thing to do is range-check seg,
not headcount as we do now: these will be different
if guest chains s/g descriptors (this does not
happen now, but we can not trust the guest).

Long term, we should add BUG_ON checks to verify
two other indexes are what we think they should be.

Reported-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---

Dave, I'll queue this on my tree, no need to bother.

 drivers/vhost/net.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 6400cd5..f095de6 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -245,7 +245,7 @@ static int get_rx_bufs(struct vhost_virtqueue *vq,
 	int r, nlogs = 0;
 
 	while (datalen > 0) {
-		if (unlikely(headcount >= VHOST_NET_MAX_SG)) {
+		if (unlikely(seg >= VHOST_NET_MAX_SG)) {
 			r = -ENOBUFS;
 			goto err;
 		}
-- 
1.7.3.rc1.5.ge5969

Tested-by: Jason Wang <jasowang at redhat.com>

----- "Michael S. Tsirkin" <mst at redhat.com> wrote:
> In mergeable buffer case, we use headcount, log_num
> and seg as indexes in same-size arrays, and
> we know that headcount <= seg and
> log_num equals either 0 or seg.
> 
> Therefore, the right thing to do is range-check seg,
> not headcount as we do now: these will be different
> if guest chains s/g descriptors (this does not
> happen now, but we can not trust the guest).
> 
> Long term, we should add BUG_ON checks to verify
> two other indexes are what we think they should be.
> 
> Reported-by: Jason Wang <jasowang at redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
> 
> Dave, I'll queue this on my tree, no need to bother.
> 
>  drivers/vhost/net.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 6400cd5..f095de6 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -245,7 +245,7 @@ static int get_rx_bufs(struct vhost_virtqueue
> *vq,
>  	int r, nlogs = 0;
>  
>  	while (datalen > 0) {
> -		if (unlikely(headcount >= VHOST_NET_MAX_SG)) {
> +		if (unlikely(seg >= VHOST_NET_MAX_SG)) {
>  			r = -ENOBUFS;
>  			goto err;
>  		}
> -- 
> 1.7.3.rc1.5.ge5969
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