netdev - Re: [Patch net] socket: close race condition between sock_close() and sockfs

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Message-ID: <CAM_iQpUpBYC0MpjOsYLn5ZnsjS3pSHbZo2AE=trfrsJNxQYyxA@mail.gmail.com>
Date:   Thu, 7 Jun 2018 14:45:58 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        shankarapailoor <shankarapailoor@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [Patch net] socket: close race condition between sock_close() and sockfs_setattr()

On Thu, Jun 7, 2018 at 2:26 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Jun 07, 2018 at 01:39:49PM -0700, Cong Wang wrote:
>> fchownat() doesn't even hold refcnt of fd until it figures out
>> fd is really needed (otherwise is ignored) and releases it after
>> it resolves the path. This means sock_close() could race with
>> sockfs_setattr(), which leads to a NULL pointer dereference
>> since typically we set sock->sk to NULL in ->release().
>>
>> As pointed out by Al, this is unique to sockfs. So we can fix this
>> in socket layer by acquiring inode_lock in sock_close() and
>> checking against NULL in sockfs_setattr().
>
> That looks like a massive overkill - it's way heavier than it should be.

I don't see any other quick way to fix this. My initial thought is
to keep that refcnt until path_put(), apparently you don't like it
either.


> And it's very likely to trigger shitloads of deadlock warnings, some
> possibly even true.

I do audit the inode_lock usage in networking, I don't see any
deadlock, of course, there could be some non-networking code
uses socket API that I missed. But _generally_, socket doesn't
have a pointer to retrieve this inode.