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

[H. Peter Anvin]

On 04/23/2014 10:25 AM, Andrew Lutomirski wrote:
> On Wed, Apr 23, 2014 at 10:16 AM, H. Peter Anvin <hpa@xxxxxxxxx> 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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