linux-kernel - Re: [proc/sysctl] 1dd38979b2: BUG:kernel_NULL_pointer

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Message-ID: <Ymc1Zn8BbQMbjb58@bombadil.infradead.org>
Date:   Mon, 25 Apr 2022 16:57:26 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Julius Hemanth Pitti <jpitti@...co.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>, Ingo Molnar <mingo@...e.hu>,
        Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, Eli Cohen <elic@...dia.com>
Subject: Re: [proc/sysctl]  1dd38979b2:
 BUG:kernel_NULL_pointer_dereference,address

On Mon, Apr 25, 2022 at 04:34:34PM -0700, Andrew Morton wrote:
> On Mon, 25 Apr 2022 16:08:57 -0700 Luis Chamberlain <mcgrof@...nel.org> wrote:
> 
> > On Mon, Apr 25, 2022 at 02:46:07PM -0700, Andrew Morton wrote:
> > > On Mon, 25 Apr 2022 12:00:21 -0700 Luis Chamberlain <mcgrof@...nel.org> wrote:
> > > 
> > > > Andrew, can we drop this patch for now?
> > > 
> > > I've been sitting on (ie, forgotten about) this patch
> > > (https://lore.kernel.org/all/20200709235115.56954-1-jpitti@cisco.com/T/#u)
> > 
> > Jesh, yeah I see.
> > 
> > > for two years.  Evidently waiting for you/Kees/Ingo to provide
> > > guidance.  So sure, the need seems very unurgent so I can drop it.
> > 
> > Well Keew as OK with it, but I yeah I can't decipher the issue at this
> > point in time.
> > 
> > > However I fail to see how that patch could have caused this crash.  I'm
> > > suspecting a bisection error?
> > > 
> > > Maybe something is unwell in drivers/vdpa/vdpa_user/vduse_dev.c.
> > 
> > At a quick glance, yes it could very well by vduse_init() is messy and
> > races somehow with init, but if a race does lurk here my instincts tell
> > me this can't be the only place.
> > 
> > Not sure if leaving a patch in place more time to see how else things
> > can explode is worth it.
> 
> Confused.  Are you thinking that the above-linked patch was somehow
> involved in this crash?  If so, but how?  All it does it to permit
> unprivileged reads to four ints via proc_dointvec_minmax()?

If the priv allows for it access to dereferencing a pointer is allowed.
How that race happens though, indeed is beyond the patch's fault.

  Luis