Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 3 Dec 2014 16:38:46 -0800 | Subject | Re: [PATCH] context_tracking: Restore previous state in schedule_user |
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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
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