Re: [PATCH] vhost_net: fix double fget()

From: Al Viro <viro@zeniv.linux.org.uk>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, ebiggers@kernel.org,
	davem@davemloft.net
Subject: Re: [PATCH] vhost_net: fix double fget()
Date: Tue, 17 May 2022 22:00:03 +0000	[thread overview]
Message-ID: <YoQa4wzy9jSwDY7E@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <20220516044400-mutt-send-email-mst@kernel.org>

On Mon, May 16, 2022 at 04:44:19AM -0400, Michael S. Tsirkin wrote:
> > Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> and this is stable material I guess.

It is, except that commit message ought to be cleaned up.  Something
along the lines of

----
Fix double fget() in vhost_net_set_backend()

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.
----

Does the above sound sane for commit message?  And which tree would you
prefer it to go through?  I can take it in vfs.git#fixes, or you could
take it into your tree...