Re: [syzbot] upstream boot error: WARNING in __context_tracking_enter

[Mark Rutland]

Hi Russell,

On Fri, Mar 19, 2021 at 10:10:43AM +0000, Russell King - ARM Linux admin wrote:
> On Fri, Mar 19, 2021 at 10:54:48AM +0100, Dmitry Vyukov wrote:
> > .On Fri, Mar 19, 2021 at 10:44 AM syzbot
> > <syzbot+f09a12b2c77bfbbf51bd@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    8b12a62a Merge tag 'drm-fixes-2021-03-19' of git://anongit..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=17e815aed00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=cfeed364fc353c32
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=f09a12b2c77bfbbf51bd
> > > userspace arch: arm
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+f09a12b2c77bfbbf51bd@xxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > 
> > +Mark, arm
> > It did not get far with CONFIG_CONTEXT_TRACKING_FORCE (kernel doesn't boot).
> 
> It seems that the path:
> 
> context_tracking_user_enter()
> user_enter()
> context_tracking_enter()
> __context_tracking_enter()
> vtime_user_enter()
> 
> expects preemption to be disabled. It effectively is, because local
> interrupts are disabled by context_tracking_enter().
> 
> However, the requirement for preemption to be disabled is not
> documented... so shrug. Maybe someone can say what the real requirements
> are here.

>From dealing with this recently on arm64, theis is a bit messy. To
handle this robustly we need to do a few things in sequence, including
using the *_irqoff() variants of the context_tracking_user_*()
functions.

I wrote down the constraints in commit:
  
  23529049c6842382 ("arm64: entry: fix non-NMI user<->kernel transitions")

For user->kernel transitions, the arch code needs the following sequence
before invoking arbitrary kernel C code:

	lockdep_hardirqs_off(CALLER_ADDR0);
	user_exit_irqoff();
	trace_hardirqs_off_finish();

For kernel->user transitions, the arch code needs the following sequence
once it will no longer invoke arbitrary kernel C code, just before
returning to userspace:

	trace_hardirqs_on_prepare();
	lockdep_hardirqs_on_prepare(CALLER_ADDR0);
	user_enter_irqoff();
	lockdep_hardirqs_on(CALLER_ADDR0);

Thanks,
Mark.