On Mon, May 16, 2022 at 12:17:56PM +0800, Jason Wang wrote:
>
> 在 2022/5/15 11:30, Al Viro 写道:
> > [tun/tap and vhost folks Cc'd]
> >
> > 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 <viro@xxxxxxxxxxxxxxxxxx>
> > ---
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 792ab5f23647..86ea7695241e 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -1450,13 +1450,9 @@ 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;
> > @@ -1465,7 +1461,6 @@ static struct ptr_ring *get_tap_ptr_ring(int fd)
> > goto out;
> > ring = NULL;
> > out:
> > - fput(file);
> > return ring;
> > }
> > @@ -1553,7 +1548,7 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> > if (r)
> > goto err_used;
> > if (index == VHOST_NET_VQ_RX)
> > - nvq->rx_ring = get_tap_ptr_ring(fd);
> > + nvq->rx_ring = get_tap_ptr_ring(sock->file);
>
>
> sock could be NULL if we want to stop the vhost-net.
Can you cook up a correct patch then please?
> Other looks fine.
>
> Thanks
>
>
> > oldubufs = nvq->ubufs;
> > nvq->ubufs = ubufs;
> >
