netdev - Re: fs, net: deadlock between bind/splice on af

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Message-ID: <20170207142035.vfnyrbggb6kbfzgd@dhcp-1-212.brq.redhat.com>
Date:   Tue, 7 Feb 2017 15:20:35 +0100
From:   Mateusz Guzik <mguzik@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Rainer Weikusat <rweikusat@...ileactivedefense.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: fs, net: deadlock between bind/splice on af_unix

On Sun, Feb 05, 2017 at 11:22:12PM -0800, Cong Wang wrote:
> On Tue, Jan 31, 2017 at 10:14 AM, Mateusz Guzik <mguzik@...hat.com> wrote:
> > On Mon, Jan 30, 2017 at 10:44:03PM -0800, Cong Wang wrote:
> >> Mind being more specific?
> >
> > Consider 2 threads which bind the same socket, but with different paths.
> >
> > Currently exactly one file will get created, the one used to bind.
> >
> > With your patch both threads can succeed creating their respective
> > files, but only one will manage to bind. The other one must error out,
> > but it already created a file it is unclear what to do with.
> 
> In this case, it simply puts the path back:
> 
>         err = -EINVAL;
>         if (u->addr)
>                 goto out_up;
> [...]
> 
> out_up:
>         mutex_unlock(&u->bindlock);
> out_put:
>         if (err)
>                 path_put(&path);
> out:
>         return err;
> 
> 
> Which is what unix_release_sock() does too:
> 
>         if (path.dentry)
>                 path_put(&path);

Yes, but unix_release_sock is expected to leave the file behind.
Note I'm not claiming there is a leak, but that racing threads will be
able to trigger a condition where you create a file and fail to bind it.

What to do with the file now?

Untested, but likely a working solution would rework the code so that
e.g. a flag is set and the lock can be dropped.