Re: [PATCH/FIXUP FOR STABLE BEFORE THIS SERIES] KVM: s390: do not clobber user space fpc during guest reset

Cornelia Huck
2020-01-30T10:39:54Z

On Thu, 30 Jan 2020 10:49:35 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 30.01.20 09:55, Christian Borntraeger wrote:
> > The initial CPU reset currently clobbers the userspace fpc. This was an
> > oversight during a fixup for the lazy fpu reloading rework. The reset
> > calls are only done from userspace ioctls. No CPU context is loaded, so
> > we can (and must) act directly on the sync regs, not on the thread
> > context. Otherwise the fpu restore call will restore the zeroes fpc to
> > userspace.
> >
> > Cc: stable@kernel.org
> > Fixes: 9abc2a08a7d6 ("KVM: s390: fix memory overwrites when vx is disabled")
> > Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > ---
> > arch/s390/kvm/kvm-s390.c | 3 +--
> > 1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index c059b86..eb789cd 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -2824,8 +2824,7 @@ static void kvm_s390_vcpu_initial_reset(struct kvm_vcpu *vcpu)
> > vcpu->arch.sie_block->gcr[14] = CR14_UNUSED_32 |
> > CR14_UNUSED_33 |
> > CR14_EXTERNAL_DAMAGE_SUBMASK;
> > - /* make sure the new fpc will be lazily loaded */
> > - save_fpu_regs();
> > + vcpu->run->s.regs.fpc = 0;
> > current->thread.fpu.fpc = 0;
> > vcpu->arch.sie_block->gbea = 1;
> > vcpu->arch.sie_block->pp = 0;
> >
>
> kvm_arch_vcpu_ioctl() does a vcpu_load(vcpu), followed by the call to
> kvm_arch_vcpu_ioctl_initial_reset(), followed by a vcpu_put().
>
> What am I missing?

I have been staring at this patch for some time now, and I fear I'm
missing something as well. Can we please get more explanation?

>
> (we could get rid of the kvm_arch_vcpu_ioctl_initial_reset() wrapper)
>