KVM: s390: fix guarded storage control register handling

commit 44bada2 upstream.

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: 4e0b1ab ("KVM: s390: gs support for kvm guests")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Cc: <stable@vger.kernel.org> # 4.12
Link: https://lore.kernel.org/r/20210415080127.1061275-1-hca@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/s390/kvm/kvm-s390.c

@@ -4308,16 +4308,16 @@ 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 */
 }