linux-kernel - [PATCH] x86: Load __USER_DS into DS/ES after resume

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Date:	Sun, 14 Jun 2015 08:56:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"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: [PATCH] x86: Load __USER_DS into DS/ES after resume


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

> >> 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.

Ok, so something like the patch below, right?

Thanks,

	Ingo

=====================>
 arch/x86/kernel/acpi/wakeup_32.S | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
index 665c6b7d2ea9..7302bbaea184 100644
--- a/arch/x86/kernel/acpi/wakeup_32.S
+++ b/arch/x86/kernel/acpi/wakeup_32.S
@@ -12,11 +12,13 @@ ENTRY(wakeup_pmode_return)
 wakeup_pmode_return:
 	movw	$__KERNEL_DS, %ax
 	movw	%ax, %ss
-	movw	%ax, %ds
-	movw	%ax, %es
 	movw	%ax, %fs
 	movw	%ax, %gs
 
+	movw	$__KERNEL_DS, %ax
+	movw	%ax, %ds
+	movw	%ax, %es
+
 	# reload the gdt, as we need the full 32 bit address
 	lidt	saved_idt
 	lldt	saved_ldt
--
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