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

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

[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);
 
 		oldubufs = nvq->ubufs;
 		nvq->ubufs = ubufs;