This is a note to let you know that I've just added the patch titled Fix double fget() in vhost_net_set_backend() to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: fix-double-fget-in-vhost_net_set_backend.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From fb4554c2232e44d595920f4d5c66cf8f7d13f9bc Mon Sep 17 00:00:00 2001 From: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Date: Mon, 16 May 2022 16:42:13 +0800 Subject: Fix double fget() in vhost_net_set_backend() From: Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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; Patches currently in stable-queue which might be from viro@xxxxxxxxxxxxxxxxxx are queue-5.15/fix-double-fget-in-vhost_net_set_backend.patch