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

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Message-ID: <YoQa4wzy9jSwDY7E@zeniv-ca.linux.org.uk>
Date:   Tue, 17 May 2022 22:00:03 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ebiggers@...nel.org, davem@...emloft.net
Subject: Re: [PATCH] vhost_net: fix double fget()

On Mon, May 16, 2022 at 04:44:19AM -0400, Michael S. Tsirkin wrote:
> > Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> 
> Acked-by: Michael S. Tsirkin <mst@...hat.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...