Re: [PATCH v2] KVM: s390: do not clobber user space registers during guest reset/store status

Christian Borntraeger
2020-01-30T11:44:16Z

On 30.01.20 12:42, Christian Borntraeger wrote:
> The two ioctls for initial CPU reset and store status currently clobber
> the userspace fpc and potentially access registers. 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: e1788bb995be ("KVM: s390: handle floating point registers in the run ioctl not in vcpu_put/load")
> Fixes: 31d8b8d41a7e ("KVM: s390: handle access registers in the run ioctl not in vcpu_put/load")
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
> arch/s390/kvm/kvm-s390.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index c059b86..936415b 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;
> @@ -4343,7 +4342,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
> switch (ioctl) {
> case KVM_S390_STORE_STATUS:
> idx = srcu_read_lock(&vcpu->kvm->srcu);
> - r = kvm_s390_vcpu_store_status(vcpu, arg);
> + r = kvm_s390_vcpu_store_status_unloaded(vcpu, arg);
kvm_s390_store_status_unloaded of course.....

> srcu_read_unlock(&vcpu->kvm->srcu, idx);
> break;
> case KVM_S390_SET_INITIAL_PSW: {
>