Re: [Xen-devel] [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero

[Juergen Gross <jgross@xxxxxxxx>]

On 25/04/17 21:18, Borislav Petkov wrote:
> On Tue, Apr 25, 2017 at 08:34:34PM +0200, Juergen Gross wrote:
>> And what happens when there is a scheduling event right here?
>> __switch_to() will see X86_BUG_SYSRET_SS_ATTRS set and take a wrong
>> path.
> 
> So the whole thing we're doing right now is wrong: set bit and then
> clear bit.

Right. And this is handled by my patch.

The really clean solution would be to add this test to set_cpu_bug()
et al. Don't set/clear the bit if anyone selected to force a value.
The force variants would be capable to overwrite, the normal variants
wouldn't. This would require a lot of research to avoid pitfalls with
today's handling, though. OTOH one could remove all the calls to
apply_forced_caps().

> 
> We should not set the bit at all and there won't be any window to get it
> wrong.
> 
> So can we do something like this instead:
> 
>       if (!cpu_has(c, X86_FEATURE_XENPV))
>               set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> 
> or is XENPV the wrong thing to test?

This would work. OTOH I'd prefer to test whether the bit should be
forced to remain zero than use the knowledge _who_ is trying to force
it.


Juergen

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