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

[David Hildenbrand <david@redhat.com>]

On 30.01.20 12:14, Christian Borntraeger wrote:
> 
> 
> On 30.01.20 12:01, Christian Borntraeger wrote:
>>
>>
>> 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).
> 
> see
> e1788bb KVM: s390: handle floating point registers in the run ioctl not in vcpu_put/load
> 31d8b8d KVM: s390: handle access registers in the run ioctl not in vcpu_put/load
> 
> so maybe we want to change the Fixes tag to this patch.
> 

Yes, because

e1788bb KVM: s390: handle floating point registers in the run ioctl not
in vcpu_put/load

broke it.

We should audit all users of save_fpu_regs().


-- 
Thanks,

David / dhildenb