From 44bada28219031f9e8e86b84460606efa57b871e Mon Sep 17 00:00:00 2001
From: Heiko Carstens <hca@linux.ibm.com>
Date: Thu, 15 Apr 2021 10:01:27 +0200
Subject: KVM: s390: fix guarded storage control register handling

From: Heiko Carstens <hca@linux.ibm.com>

commit 44bada28219031f9e8e86b84460606efa57b871e 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: 4e0b1ab72b8a ("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 |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4310,16 +4310,16 @@ static void store_regs_fmt2(struct kvm_v
 	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 */
 }