Re: [PATCH] x86: Load __USER_DS into DS/ES after resume

From	Pavel Machek <pavel@ucw.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86: Load __USER_DS into DS/ES after resume
Date	2015-06-14 09:10 +0200
Message-ID	<pBadk-4Q7-7@gated-at.bofh.it> (permalink)
References	(5 earlier) <pAzns-1t4-19@gated-at.bofh.it> <pANJL-5E0-5@gated-at.bofh.it> <pAYlQ-4a9-9@gated-at.bofh.it> <pB1jH-8qP-3@gated-at.bofh.it> <pBa3E-4po-1@gated-at.bofh.it>
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On Sun 2015-06-14 08:56:35, Ingo Molnar wrote:
> 
> * Brian Gerst <brgerst@gmail.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

Umm. Are you sure? :-).
									Pavel

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[PATCH] x86: Load __USER_DS into DS/ES after resume Ingo Molnar <mingo@kernel.org> - 2015-06-14 09:00 +0200
  Re: [PATCH] x86: Load __USER_DS into DS/ES after resume Pavel Machek <pavel@ucw.cz> - 2015-06-14 09:10 +0200

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