Re: [patch 1/2] vhost: potential integer overflows

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 11, 2010 at 07:22:57PM +0200, Dan Carpenter wrote:
> I did an audit for potential integer overflows of values which get passed
> to access_ok() and here are the results.
> 
> Signed-off-by: Dan Carpenter <error27@gmail.com>
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index dd3d6f7..c2aa12c 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -429,6 +429,14 @@ static int vq_access_ok(unsigned int num,
>  			struct vring_avail __user *avail,
>  			struct vring_used __user *used)
>  {
> +
> +	if (num > UINT_MAX / sizeof *desc)
> +		return 0;
> +	if (num > UINT_MAX / sizeof *avail->ring - sizeof *avail)
> +		return 0;
> +	if (num > UINT_MAX / sizeof *used->ring - sizeof *used)
> +		return 0;
> +
>  	return access_ok(VERIFY_READ, desc, num * sizeof *desc) &&
>  	       access_ok(VERIFY_READ, avail,
>  			 sizeof *avail + num * sizeof *avail->ring) &&
> @@ -447,6 +455,9 @@ int vhost_log_access_ok(struct vhost_dev *dev)
>  /* Caller should have vq mutex and device mutex */
>  static int vq_log_access_ok(struct vhost_virtqueue *vq, void __user *log_base)
>  {
> +	if (vq->num > UINT_MAX / sizeof *vq->used->ring - sizeof *vq->used)
> +		return 0;
> +
>  	return vq_memory_access_ok(log_base, vq->dev->memory,
>  			    vhost_has_feature(vq->dev, VHOST_F_LOG_ALL)) &&
>  		(!vq->log_used || log_access_ok(log_base, vq->log_addr,
> @@ -606,12 +617,17 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
>  			}
>  
>  			/* Also validate log access for used ring if enabled. */
> -			if ((a.flags & (0x1 << VHOST_VRING_F_LOG)) &&
> -			    !log_access_ok(vq->log_base, a.log_guest_addr,
> +			if (a.flags & (0x1 << VHOST_VRING_F_LOG)) {
> +				if (vq->num > UINT_MAX / sizeof *vq->used->ring - sizeof *vq->used) {
> +					r = -EINVAL;
> +					break;
> +				}
> +				if (!log_access_ok(vq->log_base, a.log_guest_addr,
>  					   sizeof *vq->used +
>  					   vq->num * sizeof *vq->used->ring)) {
> -				r = -EINVAL;
> -				break;
> +					r = -EINVAL;
> +					break;
> +				}
>  			}
>  		}
>  


As far as I can see, maximum value for num is 64K - 1:

                if (!s.num || s.num > 0xffff || (s.num & (s.num - 1))) {
                        r = -EINVAL;
                        break;
                }

How can any of the above two trigger?
It seems easier to check value for sanity at a single place where it's
passed from userspace to kernel.

-- 
MST