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KVM: s390: fix guarded storage control register handling 

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: 4e0b1ab72b ("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>
This commit is contained in:
Heiko Carstens 2021-04-15 10:01:27 +02:00 committed by Christian Borntraeger
parent c3171e94cc
commit 44bada2821
1 changed files with 2 additions and 2 deletions
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4
arch/s390/kvm/kvm-s390.c
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@ -4313,16 +4313,16 @@ 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 */
}