Date:	Fri, 12 Jun 2015 08:07:47 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:	mingo@...hat.com, tglx@...utronix.de, hpa@...or.com, pavel@....cz,
	rjw@...ysocki.net, x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: General protection fault after STR (32 bit systems
 only)


* Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> wrote:

> Suspend to RAM process is returning to userspsace with DS set to KERNEL_DS
> after resume, this cause general protection fault. [...]

But s2ram has no influence on 'returning to user-space'. So unless I'm missing 
something this changelog makes no sense.

Unfortunately the patch seems to be completely bogus as well:

> [...] This is very difficult to reproduce, but this does happen on 32 bit 
> systems. This crash is not reproduced after save/restore DS during calls to 
> _save_processor_state/ __restore_processor_state respectively.
>
> Similar issue was reported in the past
> https://bugzilla.kernel.org/show_bug.cgi?id=61781, for which the root cause
> was not identified.
> 
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/include/asm/suspend_32.h | 2 +-
>  arch/x86/power/cpu.c              | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index 552d6c9..3503d0b 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -11,7 +11,7 @@
>  
>  /* image of the saved processor state */
>  struct saved_context {
> -	u16 es, fs, gs, ss;
> +	u16 ds, es, fs, gs, ss;
>  	unsigned long cr0, cr2, cr3, cr4;
>  	u64 misc_enable;
>  	bool misc_enable_saved;
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index 757678f..e0dfb01 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -79,6 +79,7 @@ static void __save_processor_state(struct saved_context *ctxt)
>  	 * segment registers
>  	 */
>  #ifdef CONFIG_X86_32
> +	savesegment(ds, ctxt->ds);
>  	savesegment(es, ctxt->es);
>  	savesegment(fs, ctxt->fs);
>  	savesegment(gs, ctxt->gs);
> @@ -198,6 +199,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
>  	 * segment registers
>  	 */
>  #ifdef CONFIG_X86_32
> +	loadsegment(ds, ctxt->ds);
>  	loadsegment(es, ctxt->es);
>  	loadsegment(fs, ctxt->fs);
>  	loadsegment(gs, ctxt->gs);

So save_processor_state() is used by 32-bit s2ram in the following place:

arch/x86/kernel/acpi/wakeup_32.S:       call    save_processor_state

Other uses are:

arch/x86/kernel/acpi/wakeup_64.S:       call    save_processor_state
arch/x86/kernel/apm_32.c:       save_processor_state();
arch/x86/kernel/machine_kexec_32.c:             save_processor_state();
arch/x86/kernel/machine_kexec_64.c:             save_processor_state();
arch/x86/platform/olpc/xo1-wakeup.S:    call    save_processor_state
kernel/power/hibernate.c:       save_processor_state();
kernel/power/hibernate.c:       save_processor_state();

but neither of these call sites seems to matter to the bug: the 32-bit system does 
not use APM, kexec, is not an OLPC and does not use hibernation.

And if we look at arch/x86/kernel/acpi/wakeup_32.S it's a straightforward full 
state save/restore before ACPI (BIOS) downcalls.

Furthermore, the bug report in:

    https://bugzilla.kernel.org/show_bug.cgi?id=61781

talks about SIGSEGVs, and claims that this regression triggers in v3.11 but does 
not trigger in v3.10.

1)

So the first critical question is: if the ACPI/BIOS suspend code corrupts the 
kernel's DS, how can we get so far as to resume fully, return to user-space, and 
segfault there so that it can all be reported?

So neither the explanation nor the code makes any sense in the context of the 
reported bugs. Can anyone else offer any plausible theory about why this patch 
would fix 32-bit user-space segfaults?

2)

Btw., I don't mind the idea of the patch itself per se: saving/restoring DS is 
prudent to avoid the BIOS stomping on our DS.

But the restoration done in this patch is bogus, it's done way too late in a C 
function, there's a number of places where we might use the kernel DS before it's 
restored, such as restore_registers():

restore_registers:
        movl    saved_context_ebp, %ebp
        movl    saved_context_ebx, %ebx
        movl    saved_context_esi, %esi
        movl    saved_context_edi, %edi
        pushl   saved_context_eflags
        popfl
        ret

this is called before restore_processor_state().

those 'saved_context_*' references are already using the kernel DS! So if it's 
corrupted, we'll crash there already. So your patch seems totally nonsensical.

3)

So the patch below (totally untested) does the DS restoration early on, as the 
first thing after we emerge from the sleep. This should be equivalent to your 
patch, but is more robust and much simpler: there's no need to save DS, because we 
know that it has to be __KERNEL_DS.

Could you please test whether this solves the problem as well? Also, could you 
please describe how the failure triggers in your system: how many times do you 
have to suspend/resume to trigger the segfaults, and is there anything that makes 
the failures less or more likely?

Thanks,

	Ingo

=================>
 arch/x86/kernel/acpi/wakeup_32.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
index 665c6b7d2ea9..9a3ce66e0dd8 100644
--- a/arch/x86/kernel/acpi/wakeup_32.S
+++ b/arch/x86/kernel/acpi/wakeup_32.S
@@ -81,6 +81,10 @@ ENTRY(do_suspend_lowlevel)
 	jmp	ret_point
 	.p2align 4,,7
 ret_point:
+	/* In case the BIOS corrupted DS, make the kernel context minimally functional: */
+	movl	$__KERNEL_DS, %eax
+	movl	%eax, %ds
+
 	call	restore_registers
 	call	restore_processor_state
 	ret
--
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