linux-kernel - Re: general protection fault in kernfs_kill

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Date:   Fri, 20 Apr 2018 10:55:13 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Al Viro <viro@...IV.linux.org.uk>,
        syzbot <syzbot+151de3f2be6b40ac8026@...kaller.appspotmail.com>,
        gregkh@...uxfoundation.org, kstewart@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        pombredanne@...b.com, syzkaller-bugs@...glegroups.com,
        tglx@...utronix.de, linux-fsdevel@...r.kernel.org
Subject: Re: general protection fault in kernfs_kill_sb

On Fri, Apr 20, 2018 at 09:31:58AM +0200, Michal Hocko wrote:
> On Fri 20-04-18 14:29:39, Tetsuo Handa wrote:
> > Eric Biggers wrote:
> > > But, there is still a related bug: when mounting sysfs, if register_shrinker()
> > > fails in sget_userns(), then kernfs_kill_sb() gets called, which frees the
> > > 'struct kernfs_super_info'.  But, the 'struct kernfs_super_info' is also freed
> > > in kernfs_mount_ns() by:
> > > 
> > >         sb = sget_userns(fs_type, kernfs_test_super, kernfs_set_super, flags,
> > >                          &init_user_ns, info);
> > >         if (IS_ERR(sb) || sb->s_fs_info != info)
> > >                 kfree(info);
> > >         if (IS_ERR(sb))
> > >                 return ERR_CAST(sb);
> > > 
> > > I guess the problem is that sget_userns() shouldn't take ownership of the 'info'
> > > if it returns an error -- but, it actually does if register_shrinker() fails,
> > > resulting in a double free.
> > > 
> > > Here is a reproducer and the KASAN splat.  This is on Linus' tree (87ef12027b9b)
> > > with vfs/for-linus merged in.
> > 
> > I'm waiting for response from Michal Hocko regarding
> > http://lkml.kernel.org/r/201804111909.EGC64586.QSFLFJFOVHOOtM@I-love.SAKURA.ne.jp .
> 
> I didn't plan to respond util all the Al's concerns with the existing
> scheme are resolved. This is not an urgent thing to fix so better fix it
> properly. Your API change is kinda ugly so it would be preferable to do
> it properly as suggested by Al. Maybe that will be more work but my
> understanding is that the resulting code would be better. If that is not
> the case then I do not really have any fundamental objection to your
> patch except it is ugly.

Okay, the fix was merged already as commit 8e04944f0ea8b8 ("mm,vmscan: Allow
preallocating memory for register_shrinker().").  Thanks Tetsuo!

- Eric