linux-kernel - Re: general protection fault in wb

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Message-Id: <201806080231.w582VIRn021009@www262.sakura.ne.jp>
Date:   Fri, 08 Jun 2018 11:31:18 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        syzbot <syzbot+4a7438e774b21ddd8eca@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        linux-block@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: general protection fault in wb_workfn (2)

Dmitry Vyukov wrote:
> On Tue, Jun 5, 2018 at 3:45 PM, Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
> > Dmitry, can you assign VM resources for a git tree for this bug? This bug wants to fight
> > against https://github.com/google/syzkaller/blob/master/docs/syzbot.md#no-custom-patches ...
> 
> Hi Tetsuo,
> 
> Most of the reasons for not doing it still stand. A syzkaller instance
> will produce not just this bug, it will produce hundreds of different
> bugs. Then the question is: what to do with these bugs? Report all to
> mailing lists?

Is it possible to add linux-next.git tree as a target for fuzzing? If yes,
we can try debug patches easily, in addition to find bugs earlier than now.

> I think the solution here is just to run syzkaller instance locally.
> It's just a program anybody can run it on any kernel with any custom
> patches. Moreover for local instance it's also possible to limit set
> of tested syscalls to increase probability of hitting this bug and at
> the same time filter out most of other bugs.

If this bug is reproducible with VM resources individual developer can afford...

Since my Linux development environment is VMware guests on a Windows PC, I can't
run VM instance which needs KVM acceleration. Also, due to security policy, I can't
utilize external VM resources available on the Internet, as well as I can't use ssh
and git protocols. Speak of this bug, even with a lot of VM instances, syzbot can
reproduce this bug only once or twice per a day. Thus, the question for me boils
down to, whether I can reproduce this bug using one VMware guest instance with 4GB
of memory. Effectively, I don't have access to environments for running syzkaller
instance...

> 
> Do we have any idea about the guilty subsystem? You mentioned
> bdi_unregister, why? What would be the set of syscalls to concentrate
> on?
> I will do a custom run when I get around to it, if nobody else beats me to it.

Because bdi_unregister() does "bdi->dev = NULL;" which wb_workfn() is hitting
NULL pointer dereference.