linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Date:	Wed, 23 Apr 2014 10:28:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <amluto@...il.com>
CC:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

On 04/23/2014 10:25 AM, Andrew Lutomirski wrote:
> On Wed, Apr 23, 2014 at 10:16 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 04/23/2014 10:08 AM, Andrew Lutomirski wrote:
>>>
>>> The only way I can see to trigger the race is with sigreturn, but it's
>>> still there.  Sigh.
>>
>> I don't see why sigreturn needs to be involved... all you need is
>> modify_ldt() on one CPU while the other is in the middle of an IRET
>> return.  Small window, so hard to hit, but still.
> 
> If you set the flag as soon as anyone calls modify_ldt, before any
> descriptor is installed, then I don't think this can happen.  But
> there's still sigreturn, and I don't think this is worth all the
> complexity to save a single branch on #GP.
> 

Who cares?  Since we only need to enter the fixup path for LDT
selectors, anything that is dependent on having called modify_ldt() is
already redundant.

In some ways that is the saving grace.  SS being an LDT selector is
fortunately a rare case.

> I do mean intra-kernel.  And yes, this has nothing to do with espfix,
> but it would make write_msr_safe fail more quickly :)

And, pray tell, how important is that?

	-hpa


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