Re: [PATCH 14/22] x86/fpu: Eager switch PKRU state

[Sebastian Andrzej Siewior]

On 2019-03-11 12:06:05 [+0100], To Dave Hansen wrote:
> On 2019-03-08 11:01:25 [-0800], Dave Hansen wrote:
> > On 3/8/19 10:08 AM, Sebastian Andrzej Siewior wrote:
> > > On 2019-02-25 10:16:24 [-0800], Dave Hansen wrote:
> > >>> +	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
> > >>> +		return;
> > >>> +
> > >>> +	if (current->mm) {
> > >>> +		pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
> > >>> +		WARN_ON_ONCE(!pk);
> â
> > Nothing will break, but the warning will trigger, which isn't nice.
> 
> the warning should trigger if something goes south, I was not expecting
> it to happen.
> 
> > > My understanding is that the in-kernel XSAVE will always save everything
> > > so we should never "lose" the XFEATURE_PKRU no matter what user space
> > > does.
> > > 
> > > So as test case you want
> > > 	xsave (-1 & ~XFEATURE_PKRU)
> > > 	xrestore (-1 & ~XFEATURE_PKRU)
> > > 
> > > in userland and then a context switch to see if the warning above
> > > triggers?
> > 
> > I think you need an XRSTOR with RFBM=-1 (or at least with the PKRU bit
> > set) and the PKRU bit in the XFEATURES field in the XSAVE buffer set to 0.
> 
> let me check that, write a test case in userland and I come back with
> the results. I can remove that warning but I wasn't expecting it to
> trigger so let me verify that first.

so I made dis:
	https://breakpoint.cc/tc-xsave.c

and it doesn't trigger. 
XSAVE saves what is specified and enabled. XRSTOR restores what is
specified, available in the header and enabled. Which means even if
userland disables a bit in the header, it is still available during
context switch by kernel's XSAVE as long as it is set in XSTATE_BV.
The user can't use XSETBV (can only query via XGETBV) which means that
XFEATURE_PKRU can't be removed by the user.

But you got me thinking: During signal delivery we save tasks' FPU
content on the signal stack. If the signal handler removes
XFEATURE_PKRU bit then __fpu__restore_sig() will load the "missing
state" from init_fpstate. Which means that the protection key will be
set to zero. Not sure we want this or not but this the case. A XRSTOR in
userland without XFEATURE_PKRU would leave the PKRU state unchanged.

The test case from above does this if you want to double check.

Sebastian