linux-kernel - Re: [ 05/48] x86_64, switch_to(): Load TLS descriptors before switching DS and ES

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Date:	Fri, 15 May 2015 15:31:44 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Willy Tarreau <w@....eu>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [ 05/48] x86_64, switch_to(): Load TLS descriptors before
 switching DS and ES

On Fri, 2015-05-15 at 15:25 +0100, Ben Hutchings wrote:
> On Fri, 2015-05-15 at 15:38 +0200, Willy Tarreau wrote:
> > Hi Ben,
> > 
> > On Fri, May 15, 2015 at 01:32:20PM +0100, Ben Hutchings wrote:
> > > On Fri, 2015-05-15 at 10:05 +0200, Willy Tarreau wrote:
> > > > 2.6.32-longterm review patch.  If anyone has any objections, please let me know.
> > > > 
> > > > ------------------
> > > > 
> > > > From: Andy Lutomirski <luto@...capital.net>
> > > > 
> > > > commit f647d7c155f069c1a068030255c300663516420e upstream.
> > > > 
> > > > Otherwise, if buggy user code points DS or ES into the TLS
> > > > array, they would be corrupted after a context switch.
> > > > 
> > > > This also significantly improves the comments and documents some
> > > > gotchas in the code.
> > > > 
> > > > Before this patch, the both tests below failed.  With this
> > > > patch, the es test passes, although the gsbase test still fails.
> > > [...]
> > > 
> > > This depends on the changes to FPU/MMX/SSE state management that you
> > > didn't apply to 2.6.32.  Note this comment:
> > > 
> > > 	/* Must be after DS reload */
> > > 	unlazy_fpu(prev_p);
> > 
> > Are you sure you're not confusing with another one ? When running
> > estest without this patch, I get "FAIL: ES corrupted 1000/1000 times"
> > while I get "OK: ES was preserved" once applied, so it does seem to
> > do what it's intended for.
> >
> > Also I'm not seeing any reference to the comment above in the patch
> > nor around it, which leaves me confused :-/
> 
> v2.6.32.65:arch/x86/kernel/process_64.c:425:    /* Must be after DS reload */
> 
> If this comment is correct then the patch will cause a regression for
> FPU state management.  The comment was introduced by:
> 
> commit 0a5ace2ab08d45cd78d7ef0067cdcd5c812ac54f
> Author: Andi Kleen <ak@...e.de>
> Date:   Thu Oct 5 18:47:22 2006 +0200
> 
>     [PATCH] x86-64: Fix FPU corruption

And that replaced a longer comment that said "the AMD workaround
requires it to be after DS reload".  The comment above clear_fpu_state()
says:

/* AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
   is pending. Clear the x87 state here by setting it to fixed
   values. The kernel data segment can be sometimes 0 and sometimes
   new user value. Both should be ok.
   Use the PDA as safe address because it should be already in L1. */

Hopefully Andi can explain further if needed; I have no idea.

Ben.

-- 
Ben Hutchings
It is impossible to make anything foolproof because fools are so ingenious.

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