Re: [PATCH] KVM: s390: fix guarded storage control register handling

[Janosch Frank <frankja@linux.ibm.com>]

On 4/15/21 10:01 AM, Heiko Carstens wrote:
> store_regs_fmt2() has an ordering problem: first the guarded storage
> facility is enabled on the local cpu, then preemption disabled, and
> then the STGSC (store guarded storage controls) instruction is
> executed.
> 
> If the process gets scheduled away between enabling the guarded
> storage facility and before preemption is disabled, this might lead to
> a special operation exception and therefore kernel crash as soon as
> the process is scheduled back and the STGSC instruction is executed.
> 
> Fixes: 4e0b1ab72b8a ("KVM: s390: gs support for kvm guests")
> Cc: <stable@vger.kernel.org> # 4.12
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> ---
>  arch/s390/kvm/kvm-s390.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 2f09e9d7dc95..24ad447e648c 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -4307,16 +4307,16 @@ static void store_regs_fmt2(struct kvm_vcpu *vcpu)
>  	kvm_run->s.regs.bpbc = (vcpu->arch.sie_block->fpf & FPF_BPBC) == FPF_BPBC;
>  	kvm_run->s.regs.diag318 = vcpu->arch.diag318_info.val;
>  	if (MACHINE_HAS_GS) {
> +		preempt_disable();
>  		__ctl_set_bit(2, 4);
>  		if (vcpu->arch.gs_enabled)
>  			save_gs_cb(current->thread.gs_cb);
> -		preempt_disable();
>  		current->thread.gs_cb = vcpu->arch.host_gscb;
>  		restore_gs_cb(vcpu->arch.host_gscb);
> -		preempt_enable();
>  		if (!vcpu->arch.host_gscb)
>  			__ctl_clear_bit(2, 4);
>  		vcpu->arch.host_gscb = NULL;
> +		preempt_enable();
>  	}
>  	/* SIE will save etoken directly into SDNX and therefore kvm_run */
>  }
>