Subject: Re: [PATCH] vhost_net: fix double fget()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 16 May 2022 04:44:19 -0400

On Mon, May 16, 2022 at 04:42:13PM +0800, Jason Wang wrote:
> From: Al Viro <[email-masked]>
> 
> Here's another piece of code assuming that repeated fget() will yield the
> same opened file: in vhost_net_set_backend() we have
> 
>         sock = get_socket(fd);
>         if (IS_ERR(sock)) {
>                 r = PTR_ERR(sock);
>                 goto err_vq;
>         }
> 
>         /* start polling new socket */
>         oldsock = vhost_vq_get_backend(vq);
>         if (sock != oldsock) {
> ...
>                 vhost_vq_set_backend(vq, sock);
> ...
>                 if (index == VHOST_NET_VQ_RX)
>                         nvq->rx_ring = get_tap_ptr_ring(fd);
> 
> with
> static struct socket *get_socket(int fd)
> {
>         struct socket *sock;
> 
>         /* special case to disable backend */
>         if (fd == -1)
>                 return NULL;
>         sock = get_raw_socket(fd);
>         if (!IS_ERR(sock))
>                 return sock;
>         sock = get_tap_socket(fd);
>         if (!IS_ERR(sock))
>                 return sock;
>         return ERR_PTR(-ENOTSOCK);
> }
> and
> static struct ptr_ring *get_tap_ptr_ring(int fd)
> {
>         struct ptr_ring *ring;
>         struct file *file = fget(fd);
> 
>         if (!file)
>                 return NULL;
>         ring = tun_get_tx_ring(file);
>         if (!IS_ERR(ring))
>                 goto out;
>         ring = tap_get_ptr_ring(file);
>         if (!IS_ERR(ring))
>                 goto out;
>         ring = NULL;
> out:
>         fput(file);
>         return ring;
> }
> 
> Again, there is no promise that fd will resolve to the same thing for
> lookups in get_socket() and in get_tap_ptr_ring().  I'm not familiar
> enough with the guts of drivers/vhost to tell how easy it is to turn
> into attack, but it looks like trouble.  If nothing else, the pointer
> returned by tun_get_tx_ring() is not guaranteed to be pinned down by
> anything - the reference to sock will _usually_ suffice, but that
> doesn't help any if we get a different socket on that second fget().
> 
> One possible way to fix it would be the patch below; objections?
> 
> Signed-off-by: Al Viro <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

and this is stable material I guess.

> ---
>  drivers/vhost/net.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 28ef323882fb..0bd7d91de792 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1449,13 +1449,9 @@ static struct socket *get_raw_socket(int fd)
>  	return ERR_PTR(r);
>  }
>  
> -static struct ptr_ring *get_tap_ptr_ring(int fd)
> +static struct ptr_ring *get_tap_ptr_ring(struct file *file)
>  {
>  	struct ptr_ring *ring;
> -	struct file *file = fget(fd);
> -
> -	if (!file)
> -		return NULL;
>  	ring = tun_get_tx_ring(file);
>  	if (!IS_ERR(ring))
>  		goto out;
> @@ -1464,7 +1460,6 @@ static struct ptr_ring *get_tap_ptr_ring(int fd)
>  		goto out;
>  	ring = NULL;
>  out:
> -	fput(file);
>  	return ring;
>  }
>  
> @@ -1551,8 +1546,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  		r = vhost_net_enable_vq(n, vq);
>  		if (r)
>  			goto err_used;
> -		if (index == VHOST_NET_VQ_RX)
> -			nvq->rx_ring = get_tap_ptr_ring(fd);
> +		if (index == VHOST_NET_VQ_RX) {
> +			if (sock)
> +				nvq->rx_ring = get_tap_ptr_ring(sock->file);
> +			else
> +				nvq->rx_ring = NULL;
> +		}
>  
>  		oldubufs = nvq->ubufs;
>  		nvq->ubufs = ubufs;
> -- 
> 2.25.1