Re: PROBLEM: Regression of MMU causing guest VM application errors

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Nov 27, 2019 at 07:24:09AM -0800, Sean Christopherson wrote:
> On Wed, Nov 20, 2019 at 11:28:43AM -0800, Sean Christopherson wrote:
> > On Wed, Nov 20, 2019 at 02:04:38PM -0500, Derek Yerger wrote:
> > > 
> > > > Debug patch attached.  Hopefully it finds something, it took me an
> > > > embarassing number of attempts to get correct, I kept screwing up checking
> > > > a bit number versus checking a bit mask...
> > > > <0001-thread_info-Add-a-debug-hook-to-detect-FPU-changes-w.patch>
> > > 
> > > Should this still be tested despite Wanpeng Li’s comments that the issue may
> > > have been fixed in a 5.3 release candidate?
> > 
> > Yes.
> > 
> > The actual bug fix, commit e751732486eb3 (KVM: X86: Fix fpu state crash in
> > kvm guest), is present in v5.2.7.
> > 
> > Unless there's a subtlety I'm missing, commit d9a710e5fc4941 (KVM: X86:
> > Dynamically allocate user_fpu) is purely an optimization and should not
> > have a functional impact.

Any update on this?  Syzkaller also appears to be hitting this[*], but it
hasn't been able to generate a reproducer.

[*] https://syzkaller.appspot.com/bug?extid=00be5da1d75f1cc95f6b


> ---
> 
> Any chance the below change fixes your issue?  It's a bug fix for AVX
> corruption during signal delivery[*].  It doesn't seem like the same thing
> you are seeing, but it's worth trying.
> 
> [*] https://lkml.kernel.org/r/20191127124243.u74osvlkhcmsskng@linutronix.de/
> 
>  arch/x86/include/asm/fpu/internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 4c95c365058aa..44c48e34d7994 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -509,7 +509,7 @@ static inline void __fpu_invalidate_fpregs_state(struct fpu *fpu)
>  
>  static inline int fpregs_state_valid(struct fpu *fpu, unsigned int cpu)
>  {
> -	return fpu == this_cpu_read_stable(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
> +	return fpu == this_cpu_read(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
>  }