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

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Message-ID: <CAM_iQpWu+V2E_uxJLUjS+xM+94CKKR+Bv95UrFPuswUAuAGM+w@mail.gmail.com>
Date:   Tue, 17 Jan 2017 13:21:48 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     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 Mon, Jan 16, 2017 at 1:32 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Fri, Dec 9, 2016 at 7:41 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> 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?
>
>
> Ping. This is still happening on HEAD.
>

Thanks for your reminder. Mind to give the attached patch (compile only)
a try? I take another approach to fix this deadlock, which moves the
unix_mknod() out of unix->bindlock. Not sure if there is any unexpected
impact with this way.

Thanks.

View attachment "unix.diff" of type "text/plain" (1678 bytes)