Re: [PATCH] context_tracking: Restore previous state in schedule_user

Andy Lutomirski
2014-12-04T00:39:10Z

On Wed, Dec 3, 2014 at 4:30 PM, Dave Jones <davej@redhat.com> wrote:
> On Wed, Dec 03, 2014 at 04:04:31PM -0800, Andy Lutomirski wrote:
> > On Wed, Dec 3, 2014 at 3:58 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > On Wed, Dec 03, 2014 at 03:18:41PM -0800, Andy Lutomirski wrote:
> > >> It appears that some SCHEDULE_USER (asm for schedule_user) callers
> > >> in arch/x86/kernel/entry_64.S are called from RCU kernel context,
> > >> and schedule_user will return in RCU user context. This causes RCU
> > >> warnings and possible failures.
> > >>
> > >> This is intended to be a minimal fix suitable for 3.18.
> > >>
> > >> Reported-by: Dave Jones <davej@redhat.com>
> > >> Cc: Oleg Nesterov <oleg@redhat.com>
> > >> Cc: Frédéric Weisbecker <fweisbec@gmail.com>
> > >> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> > >> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> > >
> > > Ah, we sent it about at the same time :-)
> > >
> > > Might be too late for 3.18 though because it's not a regression.
>
> Wait, so how come that trace didn't start showing up until recently ?

Looking at the code, it's because int_careful has the same bug, but
syscall_trace_leave does:

/*
* We may come here right after calling schedule_user()
* or do_notify_resume(), in which case we can be in RCU
* user mode.
*/
user_exit();

which means that this issue was anticipated when that comment was written.

Prior to the 3.18 seccomp changes and the _TIF_WORK typo fix, it would
have been difficult to hit sysret_audit when context tracking was on
(you could do it once on the way out from a syscall that enabled
context tracking). So this is 3.18 regression.

The sysret_audit code is still totally screwed up AFAICT. At the very
least, the whole mess rather strongly suggests that, if both context
tracking and audit are on, then __audit_syscall_exit is called *twice*
on each syscall. __audit_syscall_exit seems to be idempotent, so
maybe no one has noticed that little glitch.

I'll ask the x86 people to include my sysret_audit removal for 3.19,
since I think that this schedule_user change is a better last-minute
fix than removing a whole chunk of asm.

--Andy

>
> Dave
>

--
Andy Lutomirski
AMA Capital Management, LLC