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

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Message-ID: <CALCETrX_goAyqMVdkZTnJwDCJgfz2osVVsNAt8Gv3ZqZMX85ZQ@mail.gmail.com>
Date:	Sat, 13 Jun 2015 11:23:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Brian Gerst <brgerst@...il.com>, "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 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.

--Andy
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