Re: [PATCH] x86/fpu/xstate: Fix xcomp_bv in XSAVES header

[Joakim Tjernlund <Joakim.Tjernlund@infinera.com>]

On Tue, 2017-01-24 at 09:09 +0100, Ingo Molnar wrote:
> * Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:

This needs to go to stable too, 4.8 and 4.9

 Jocke

> 
> > The compacted-format XSAVES area is determined at boot time and
> > never changed after.  The field xsave.header.xcomp_bv indicates
> > which components are in the fixed XSAVES format.
> > 
> > In fpstate_init() we did not set xcomp_bv to reflect the XSAVES
> > format since at the time there is no valid data.
> > 
> > However, after we do copy_init_fpstate_to_fpregs() in fpu__clear(),
> > as in commit: b22cbe404a9cc3c7949e380fa1861e31934c8978, and when
> > __fpu_restore_sig() does fpu__restore() for a COMPAT-mode app,
> > a #GP occurs.  This can be easily triggered by doing valgrind on
> > a COMPAT-mode "Hello World," as reported by Joakim Tjernlund and
> > others:
> > 
> > 	https://bugzilla.kernel.org/show_bug.cgi?id=190061
> > 
> > Fix it by setting xcomp_bv correctly.
> > 
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > Reported-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
> > ---
> >  arch/x86/kernel/fpu/core.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index c289e2f..e540dc1 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -9,6 +9,7 @@
> >  #include <asm/fpu/regset.h>
> >  #include <asm/fpu/signal.h>
> >  #include <asm/fpu/types.h>
> > +#include <asm/fpu/xstate.h>
> >  #include <asm/traps.h>
> >  
> >  #include <linux/hardirq.h>
> > @@ -235,7 +236,8 @@ void fpstate_init(union fpregs_state *state)
> >  	 * it will #GP. Make sure it is replaced after the memset().
> >  	 */
> >  	if (static_cpu_has(X86_FEATURE_XSAVES))
> > -		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
> > +		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> > +					       xfeatures_mask;
> 
> Ok, I have applied this - but it would be cleaner to go one step further and add a 
> fpstate_init_xstate() method that does this in xstate.c and hides the details from 
> arch/x86/kernel/fpu/core.c.
> 
> Similar to how the FX-state initialization is done today:
> 
> >  	if (static_cpu_has(X86_FEATURE_FXSR))
> >  		fpstate_init_fxstate(&state->fxsave);
> 
> Thanks,
> 
> 	Ingo