Linux Virtualization - Mar 2018 - [PATCH net] vhost: validate log when IOTLB is enabled

Vq log_base is the userspace address of bitmap which has nothing to do
with IOTLB. So it needs to be validated unconditionally otherwise we
may try use 0 as log_base which may lead to pin pages that will lead
unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).

Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
Reported-by: syzbot+6304bf97ef436580fede at syzkaller.appspotmail.com
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/vhost/vhost.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 5d5a9d9..5320039 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1244,14 +1244,12 @@ static int vq_log_access_ok(struct vhost_virtqueue *vq,
 /* Caller should have vq mutex and device mutex */
 int vhost_vq_access_ok(struct vhost_virtqueue *vq)
 {
-	if (vq->iotlb) {
-		/* When device IOTLB was used, the access validation
-		 * will be validated during prefetching.
-		 */
-		return 1;
-	}
-	return vq_access_ok(vq, vq->num, vq->desc, vq->avail, vq->used)
&&
-		vq_log_access_ok(vq, vq->log_base);
+	int ret = vq_log_access_ok(vq, vq->log_base);
+
+	if (ret || vq->iotlb)
+		return ret;
+
+	return vq_access_ok(vq, vq->num, vq->desc, vq->avail, vq->used);
 }
 EXPORT_SYMBOL_GPL(vhost_vq_access_ok);
 
-- 
2.7.4

On Thu, Mar 29, 2018 at 04:00:04PM +0800, Jason Wang
wrote:
> Vq log_base is the userspace address of bitmap which has nothing to do
> with IOTLB. So it needs to be validated unconditionally otherwise we
> may try use 0 as log_base which may lead to pin pages that will lead
> unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).
> 
> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> Reported-by: syzbot+6304bf97ef436580fede at syzkaller.appspotmail.com
> Signed-off-by: Jason Wang <jasowang at redhat.com>
Acked-by: Michael S. Tsirkin <mst at redhat.com>

stable material I guess.
> ---
>  drivers/vhost/vhost.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 5d5a9d9..5320039 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1244,14 +1244,12 @@ static int vq_log_access_ok(struct vhost_virtqueue
*vq,
>  /* Caller should have vq mutex and device mutex */
>  int vhost_vq_access_ok(struct vhost_virtqueue *vq)
>  {
> -	if (vq->iotlb) {
> -		/* When device IOTLB was used, the access validation
> -		 * will be validated during prefetching.
> -		 */
> -		return 1;
> -	}
> -	return vq_access_ok(vq, vq->num, vq->desc, vq->avail,
vq->used) &&
> -		vq_log_access_ok(vq, vq->log_base);
> +	int ret = vq_log_access_ok(vq, vq->log_base);
> +
> +	if (ret || vq->iotlb)
> +		return ret;
> +
> +	return vq_access_ok(vq, vq->num, vq->desc, vq->avail,
vq->used);
>  }
>  EXPORT_SYMBOL_GPL(vhost_vq_access_ok);
>  
> -- 
> 2.7.4

On Thu, Mar 29, 2018 at 04:00:04PM +0800, Jason Wang
wrote:
> Vq log_base is the userspace address of bitmap which has nothing to do
> with IOTLB. So it needs to be validated unconditionally otherwise we
> may try use 0 as log_base which may lead to pin pages that will lead
> unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).
> 
> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> Reported-by: syzbot+6304bf97ef436580fede at syzkaller.appspotmail.com
> Signed-off-by: Jason Wang <jasowang at redhat.com>
One follow-up question:

We still observe that get user pages returns 0 sometimes. While I agree
we should not pass in unvalidated addresses, isn't this worth
documenting?
	
> ---
>  drivers/vhost/vhost.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 5d5a9d9..5320039 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1244,14 +1244,12 @@ static int vq_log_access_ok(struct vhost_virtqueue
*vq,
>  /* Caller should have vq mutex and device mutex */
>  int vhost_vq_access_ok(struct vhost_virtqueue *vq)
>  {
> -	if (vq->iotlb) {
> -		/* When device IOTLB was used, the access validation
> -		 * will be validated during prefetching.
> -		 */
> -		return 1;
> -	}
> -	return vq_access_ok(vq, vq->num, vq->desc, vq->avail,
vq->used) &&
> -		vq_log_access_ok(vq, vq->log_base);
> +	int ret = vq_log_access_ok(vq, vq->log_base);
> +
> +	if (ret || vq->iotlb)
> +		return ret;
> +
> +	return vq_access_ok(vq, vq->num, vq->desc, vq->avail,
vq->used);
>  }
>  EXPORT_SYMBOL_GPL(vhost_vq_access_ok);
>  
> -- 
> 2.7.4

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Thu, 29 Mar 2018 15:48:22 +0300
> On Thu, Mar 29, 2018 at 04:00:04PM +0800, Jason Wang wrote:
>> Vq log_base is the userspace address of bitmap which has nothing to do
>> with IOTLB. So it needs to be validated unconditionally otherwise we
>> may try use 0 as log_base which may lead to pin pages that will lead
>> unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).
>> 
>> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
>> Reported-by: syzbot+6304bf97ef436580fede at syzkaller.appspotmail.com
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
> 
> Acked-by: Michael S. Tsirkin <mst at redhat.com>
> 
> stable material I guess.
Applied and queued up for -stable.

On 2018?03?29? 22:44, Michael S. Tsirkin wrote:
> On Thu, Mar 29, 2018 at 04:00:04PM +0800, Jason Wang wrote:
>> Vq log_base is the userspace address of bitmap which has nothing to do
>> with IOTLB. So it needs to be validated unconditionally otherwise we
>> may try use 0 as log_base which may lead to pin pages that will lead
>> unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).
>>
>> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
>> Reported-by:syzbot+6304bf97ef436580fede at syzkaller.appspotmail.com
>> Signed-off-by: Jason Wang<jasowang at redhat.com>
> One follow-up question:
>
> We still observe that get user pages returns 0 sometimes. While I agree
> we should not pass in unvalidated addresses, isn't this worth
> documenting?
> 	
>
Looking at get_user_pages_fast(), it has:

 ??? if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 ??? ??? ??? ??? ??? (void __user *)start, len)))
 ??? ??? return 0;

So this is expected I think.

Thanks