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

From: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: cohuck@redhat.com, david@redhat.com, frankja@linux.ibm.com,
	kvm@vger.kernel.org, linux-s390@vger.kernel.org,
	stable@kernel.org, thuth@redhat.com
Subject: Re: [PATCH v2] KVM: s390: do not clobber user space registers during guest reset/store status
Date: Thu, 30 Jan 2020 13:01:59 +0100	[thread overview]
Message-ID: <8120c228-2935-07d4-38b9-3b9c5cb8b92c@de.ibm.com> (raw)
In-Reply-To: <1580384552-7964-1-git-send-email-borntraeger@de.ibm.com>

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.

New patch description:

    KVM: s390: do not clobber registers during guest reset/store status

    The initial CPU reset clobbers the userspace fpc and the store status
    ioctl clobbers the guest acrs + fpr.  As these calls are only done via
    ioctl (and not via vcpu_run), no CPU context is loaded, so we can (and
    must) act directly on the sync regs, not on the thread context.

    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>

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