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

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Message-ID: <CACGkMEvu8eVQ5Sy0675xjhukyPRoCKnTF+t0tpXw6dsexe3v1A@mail.gmail.com>
Date:   Wed, 18 May 2022 12:10:01 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, kvm <kvm@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Eric Biggers <ebiggers@...nel.org>,
        davem <davem@...emloft.net>
Subject: Re: [PATCH] vhost_net: fix double fget()

On Wed, May 18, 2022 at 6:00 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> 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?

Yes.

> 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...
>

Consider Michael gave an ack, it would be fine if you want to take via
your tree.

Thanks