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

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Date:	Fri, 12 Jun 2015 11:48:22 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
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)

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.  The better
fix would be to set ds/es to __USER_DS there.  Also since we are in
the kernel, fs is fixed at __KERNEL_PERCPU, and gs is either
__KERNEL_STACK_CANARY or user's gs.

--
Brian Gerst
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