[PATCH] KVM: arm/arm64: Close VMID generation race

[Marc Zyngier]

On 10/04/18 16:24, Mark Rutland wrote:
> On Tue, Apr 10, 2018 at 05:05:40PM +0200, Christoffer Dall wrote:
>> On Tue, Apr 10, 2018 at 11:51:19AM +0100, Mark Rutland wrote:
>>> I think we also need to update kvm->arch.vttbr before updating
>>> kvm->arch.vmid_gen, otherwise another CPU can come in, see that the
>>> vmid_gen is up-to-date, jump to hyp, and program a stale VTTBR (with the
>>> old VMID).
>>>
>>> With the smp_wmb() and update of kvm->arch.vmid_gen moved to the end of
>>> the critical section, I think that works, modulo using READ_ONCE() and
>>> WRITE_ONCE() to ensure single-copy-atomicity of the fields we access
>>> locklessly.
>>
>> Indeed, you're right.  I would look something like this, then:
>>
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index 2e43f9d42bd5..6cb08995e7ff 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -450,7 +450,9 @@ void force_vm_exit(const cpumask_t *mask)
>>   */
>>  static bool need_new_vmid_gen(struct kvm *kvm)
>>  {
>> -	return unlikely(kvm->arch.vmid_gen != atomic64_read(&kvm_vmid_gen));
>> +	u64 current_vmid_gen = atomic64_read(&kvm_vmid_gen);
>> +	smp_rmb(); /* Orders read of kvm_vmid_gen and kvm->arch.vmid */
>> +	return unlikely(READ_ONCE(kvm->arch.vmid_gen) != current_vmid_gen);
>>  }
>>  
>>  /**
>> @@ -500,7 +502,6 @@ static void update_vttbr(struct kvm *kvm)
>>  		kvm_call_hyp(__kvm_flush_vm_context);
>>  	}
>>  
>> -	kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen);
>>  	kvm->arch.vmid = kvm_next_vmid;
>>  	kvm_next_vmid++;
>>  	kvm_next_vmid &= (1 << kvm_vmid_bits) - 1;
>> @@ -509,7 +510,10 @@ static void update_vttbr(struct kvm *kvm)
>>  	pgd_phys = virt_to_phys(kvm->arch.pgd);
>>  	BUG_ON(pgd_phys & ~VTTBR_BADDR_MASK);
>>  	vmid = ((u64)(kvm->arch.vmid) << VTTBR_VMID_SHIFT) & VTTBR_VMID_MASK(kvm_vmid_bits);
>> -	kvm->arch.vttbr = pgd_phys | vmid;
>> +	WRITE_ONCE(kvm->arch.vttbr, pgd_phys | vmid);
>> +
>> +	smp_wmb(); /* Ensure vttbr update is observed before vmid_gen update */
>> +	kvm->arch.vmid_gen = atomic64_read(&kvm_vmid_gen);
>>  
>>  	spin_unlock(&kvm_vmid_lock);
>>  }
> 
> I think that's right, yes.
> 
> We could replace the smp_{r,w}mb() barriers with an acquire of the
> kvm_vmid_gen and a release of kvm->arch.vmid_gen, but if we're really
> trying to optimize things there are larger algorithmic changes necessary
> anyhow.
> 
>> It's probably easier to convince ourselves about the correctness of
>> Marc's code using a rwlock instead, though.  Thoughts?
> 
> I believe that Marc's preference was the rwlock; I have no preference
> either way.

I don't mind either way. If you can be bothered to write a proper commit
log for this, I'll take it. What I'd really want is Shannon to indicate
whether or not this solves the issue he was seeing.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...