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

David Hildenbrand
2020-01-30T09:49:49Z

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?

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

--
Thanks,

David / dhildenb