Re: [PATCH/FIXUP FOR STABLE BEFORE THIS SERIES] KVM: s390: do not clobber user space fpc during guest reset

Christian Borntraeger
2020-01-30T11:01:25Z

On 30.01.20 10:49, David Hildenbrand wrote:
> On 30.01.20 09:55, Christian Borntraeger wrote:
>> The initial CPU reset currently clobbers the userspace fpc. This was an
>> oversight during a fixup for the lazy fpu reloading rework. The reset
>> calls are only done from userspace ioctls. No CPU context is loaded, so
>> we can (and must) act directly on the sync regs, not on the thread
>> context. Otherwise the fpu restore call will restore the zeroes fpc to
>> userspace.
>>
>> Cc: stable@kernel.org
>> Fixes: 9abc2a08a7d6 ("KVM: s390: fix memory overwrites when vx is disabled")
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>> arch/s390/kvm/kvm-s390.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index c059b86..eb789cd 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -2824,8 +2824,7 @@ static void kvm_s390_vcpu_initial_reset(struct kvm_vcpu *vcpu)
>> vcpu->arch.sie_block->gcr[14] = CR14_UNUSED_32 |
>> CR14_UNUSED_33 |
>> CR14_EXTERNAL_DAMAGE_SUBMASK;
>> - /* make sure the new fpc will be lazily loaded */
>> - save_fpu_regs();
>> + vcpu->run->s.regs.fpc = 0;
>> current->thread.fpu.fpc = 0;
>> vcpu->arch.sie_block->gbea = 1;
>> vcpu->arch.sie_block->pp = 0;
>>
>
> kvm_arch_vcpu_ioctl() does a vcpu_load(vcpu), followed by the call to
> kvm_arch_vcpu_ioctl_initial_reset(), followed by a vcpu_put().
>
> What am I missing?

vcpu_load/put does no longer reload the registers lazily. We moved that out into the
vcpu_run ioctl itself. (this avoids register reloading during schedule).