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

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Date:	Sat, 13 Jun 2015 17:30:17 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	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)

On Sat, Jun 13, 2015 at 2:23 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sat, Jun 13, 2015 at 12:03 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> * 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.
>
> True, but it still leaves the fact that we're loading __KERNEL_DS
> instead of __USER_DS, right?  So we end up in the kernel in some
> context (I have no clue what context) with __KERNEL_DS loaded.  It's
> very easy for us to inadvertently fix it: we could return to userspace
> by any means whatsoever except SYSEXIT, or we could even return back
> to some preempted kernel context.
>
> I still think we should replace __KERNEL_DS with __USER_DS in
> wakeup_pmode_return and see if the problem goes away.

I'm pretty sure that's what the problem is.  If you look at the
sysexit path, it never reloads ds/es.  It assumes they are still
__USER_DS set at sysenter.  The iret path does restore all the user
segments.

--
Brian Gerst
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