linux-kernel - Re: [PATCH] x86: General protection fault after STR (32 bit systems only)

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Date:	Sat, 13 Jun 2015 09:03:59 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, X86 ML <x86@...nel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: General protection fault after STR (32 bit systems
 only)


* Brian Gerst <brgerst@...il.com> wrote:

> On Fri, Jun 12, 2015 at 4:36 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * H. Peter Anvin <hpa@...or.com> wrote:
> >
> >> %es is used implicitly by string instructions.
> >
> > Ok, so we are probably better off reloading ES as well early, right
> > when we return from the firmware, just in case something does
> > a copy before we hit the ES restore in restore_processor_state(),
> > which is a generic C function?
> >
> > Something like the patch below?
> >
> > I also added FS/GS/SS reloading to make it complete. If this (or a variant
> > thereof, it's still totally untested) works then we can remove the segment
> > save/restore layer in __save/restore_processor_state().
> >
> > Thanks,
> >
> >         Ingo
> >
> > ===========>
> >  arch/x86/kernel/acpi/wakeup_32.S | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
> > index 665c6b7d2ea9..1376a7fc21b7 100644
> > --- a/arch/x86/kernel/acpi/wakeup_32.S
> > +++ b/arch/x86/kernel/acpi/wakeup_32.S
> > @@ -61,6 +61,19 @@ ENTRY(wakeup_pmode_return)
> >
> >
> >  restore_registers:
> > +       /*
> > +        * In case the BIOS corrupted our segment descriptors,
> > +        * reload them to clear out any shadow descriptor
> > +        * state:
> > +        */
> > +       movl    $__USER_DS, %eax
> > +       movl    %eax, %ds
> > +       movl    %eax, %es
> > +       movl    %eax, %fs
> > +       movl    %eax, %gs
> > +       movl    $__KERNEL_DS, %eax
> > +       movl    %eax, %ss
> > +
> >         movl    saved_context_ebp, %ebp
> >         movl    saved_context_ebx, %ebx
> >         movl    saved_context_esi, %esi
> 
> If you follow the convoluted flow of the calls in this file, wakeup_pmode_return 
> is the first thing called from the trampoline on resume, and that loads the data 
> segments with __KERNEL_DS. [...]

So if wakeup_pmode_return is really the first thing called then the whole premise 
of shadow descriptor corruption goes out the window: we reload all relevant 
segment registers.

Which leaves us with only two small channels through which the patch might make a 
bug go away:

  - timing, as it introduces a small delay

  - code/cache layout, as it slightly rearranges the code

... but both of these are in the 'grasping at straws' category of hypotheses 
really.

Thanks,

	Ingo
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