[PATCH 5.15 049/132] Fix double fget() in vhost_net_set_backend()

Greg Kroah-Hartman
2022-05-23T17:58:51Z

From: Al Viro <viro@zeniv.linux.org.uk>

commit fb4554c2232e44d595920f4d5c66cf8f7d13f9bc upstream.

Descriptor table is a shared resource; two fget() on the same descriptor
may return different struct file references. get_tap_ptr_ring() is
called after we'd found (and pinned) the socket we'll be using and it
tries to find the private tun/tap data structures associated with it.
Redoing the lookup by the same file descriptor we'd used to get the
socket is racy - we need to same struct file.

Thanks to Jason for spotting a braino in the original variant of patch -
I'd missed the use of fd == -1 for disabling backend, and in that case
we can end up with sock == NULL and sock != oldsock.

Cc: stable@kernel.org
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/vhost/net.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1450,13 +1450,9 @@ err:
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
goto out;
ring = NULL;
out:
- fput(file);
return ring;
}

@@ -1552,8 +1547,12 @@ static long vhost_net_set_backend(struct
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;