Re: [BUG] double fget() in vhost/net (was Re: [PATCH] vfs: move fdput() to right place in ksys_sync_file_range())

[Jason Wang <jasowang@redhat.com>]

On Mon, May 16, 2022 at 3:54 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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@zeniv.linux.org.uk>
> > > ---
> > > 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?

Sent.

Thanks

>
> > Other looks fine.
> >
> > Thanks
> >
> >
> > >             oldubufs = nvq->ubufs;
> > >             nvq->ubufs = ubufs;
> > >
>