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

David Hildenbrand
2021-04-15T08:35:08Z

On 15.04.21 10:01, 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>
> ---
> 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 */
> }
>

LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb