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

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Message-ID: <20161209064144.GZ1555@ZenIV.linux.org.uk>
Date:   Fri, 9 Dec 2016 06:41:44 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        "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 Thu, Dec 08, 2016 at 10:32:00PM -0800, Cong Wang wrote:

> > Why do we do autobind there, anyway, and why is it conditional on
> > SOCK_PASSCRED?  Note that e.g. for SOCK_STREAM we can bloody well get
> > to sending stuff without autobind ever done - just use socketpair()
> > to create that sucker and we won't be going through the connect()
> > at all.
> 
> In the case Dmitry reported, unix_dgram_sendmsg() calls unix_autobind(),
> not SOCK_STREAM.

Yes, I've noticed.  What I'm asking is what in there needs autobind triggered
on sendmsg and why doesn't the same need affect the SOCK_STREAM case?

> I guess some lock, perhaps the u->bindlock could be dropped before
> acquiring the next one (sb_writer), but I need to double check.

Bad idea, IMO - do you *want* autobind being able to come through while
bind(2) is busy with mknod?