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SubjectRe: [PATCH 3/3] x86/mm, tracing: Fix CR2 corruption
On Wed, 3 Jul 2019 22:22:31 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Jul 03, 2019 at 12:27:34PM +0200, root 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.
> >
> > Ideally we'll clean up the entry code by moving this tracing and
> > context tracking nonsense into C some day, but let's not delay fixing
> > this longer.
> >
>
> > @@ -1180,10 +1189,10 @@ idtentry xenint3 do_int3 has_error_co
> > #endif
> >
> > idtentry general_protection do_general_protection has_error_code=1
> > -idtentry page_fault do_page_fault has_error_code=1
> > +idtentry page_fault do_page_fault has_error_code=1 read_cr2=1
> >
> > #ifdef CONFIG_KVM_GUEST
> > -idtentry async_page_fault do_async_page_fault has_error_code=1
> > +idtentry async_page_fault do_async_page_fault has_error_code=1 read_cr2=1
> > #endif
>
> While going over the various idt handlers, I found that we probably also
> need read_cr2 on do_double_fault(), otherwise it is susceptible to the
> same problem.
>

BTW, do you plan on making this for stable? Even though it's rather
invasive. Or should we just apply the band-aids first, have them
backported to stable, and then put this change on top of them for
upstream?

-- Steve

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