linux-kernel - Re: [Linux 5.2.x] /sys/kernel/debug/tracing/events/power/cpu_idle/id: BUG: kernel NULL pointer dereference, address: 0000000000000000

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Date:   Sun, 11 Aug 2019 17:33:26 +0200
From:   Salvatore Bonaccorso <carnil@...ian.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-pm@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [Linux 5.2.x]
 /sys/kernel/debug/tracing/events/power/cpu_idle/id: BUG: kernel NULL pointer
 dereference, address: 0000000000000000

Thomas,

On Sun, Aug 11, 2019 at 02:58:15PM +0200, Thomas Gleixner wrote:
> Salvatore,
> 
> On Sun, 11 Aug 2019, Salvatore Bonaccorso wrote:
> > On Sun, Aug 11, 2019 at 08:22:35AM +0200, Thomas Gleixner wrote:
> > > On Sat, 10 Aug 2019, Paul Menzel wrote:
> > > > For the record. It is also reproducible with Linux 5.2.6, and trying to print
> > > > the file contents with cat already fails.
> > > > 
> > > > ```
> > > > $ sudo ls -l /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > > > -r--r--r-- 1 root root 0 Aug 10 23:05
> > > > /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > > > $ sudo cat /sys/kernel/debug/tracing/events/power/cpu_idle/id
> > > > Killed
> > > > ```
> > 
> > This seems to be related to https://bugs.debian.org/934304 (in
> > particular https://bugs.debian.org/934304#29). The mentioned patch
> > features/all/lockdown/0031-tracefs-Restrict-tracefs-when-the-kernel-is-locked-d.patch
> > is a backport of https://patchwork.kernel.org/patch/11069661/ with
> > only change that it is converted back to the non-LSM lockdown API.
> 
> So that's a debian kernel specific issue?

"yes". A kernel build without the above patch does not exhibit the
issue. So the issue is specific to that lockdown patch ("tracefs:
Restrict tracefs when the kernel is locked down").

Steven and Matthew are both CC'ed.

Regards,
Salvatore