Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption

[Andy Lutomirski]

On Sun, Jul 7, 2019 at 8:10 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Thu, Jul 4, 2019 at 1:03 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Despire the current efforts to read CR2 before tracing happens there
> > still exist a number of possible holes:
> >
> >   idtentry page_fault             do_page_fault           has_error_code=1
> >     call error_entry
> >       TRACE_IRQS_OFF
> >         call trace_hardirqs_off*
> >           #PF // modifies CR2
> >
> >       CALL_enter_from_user_mode
> >         __context_tracking_exit()
> >           trace_user_exit(0)
> >             #PF // modifies CR2
> >
> >     call do_page_fault
> >       address = read_cr2(); /* whoopsie */
> >
> > And similar for i386.
> >
> > Fix it by pulling the CR2 read into the entry code, before any of that
> > stuff gets a chance to run and ruin things.
>
> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
>
> Subject to the discussion as to whether this is the right approach at all.

FWIW, I'm leaning toward suggesting that we apply the trivial tracing
fix and backport *that*.  Then, in -tip, we could revert it and apply
this patch instead.