Re: [PATCH 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd

[Robin Murphy]

Hi Marc,

On 15/03/17 13:43, Marc Zyngier wrote:
> On 15/03/17 13:35, Christoffer Dall wrote:
>> On Wed, Mar 15, 2017 at 01:28:07PM +0000, Marc Zyngier wrote:
>>> On 15/03/17 10:56, Christoffer Dall wrote:
>>>> On Wed, Mar 15, 2017 at 09:39:26AM +0000, Marc Zyngier wrote:
>>>>> On 15/03/17 09:21, Christoffer Dall wrote:
>>>>>> On Tue, Mar 14, 2017 at 02:52:34PM +0000, Suzuki K Poulose wrote:
>>>>>>> In kvm_free_stage2_pgd() we don't hold the kvm->mmu_lock while calling
>>>>>>> unmap_stage2_range() on the entire memory range for the guest. This could
>>>>>>> cause problems with other callers (e.g, munmap on a memslot) trying to
>>>>>>> unmap a range.
>>>>>>>
>>>>>>> Fixes: commit d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
>>>>>>> Cc: stable@xxxxxxxxxxxxxxx # v3.10+
>>>>>>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>>>>>>> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>>>>> ---
>>>>>>>  arch/arm/kvm/mmu.c | 3 +++
>>>>>>>  1 file changed, 3 insertions(+)
>>>>>>>
>>>>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>>>>> index 13b9c1f..b361f71 100644
>>>>>>> --- a/arch/arm/kvm/mmu.c
>>>>>>> +++ b/arch/arm/kvm/mmu.c
>>>>>>> @@ -831,7 +831,10 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
>>>>>>>  	if (kvm->arch.pgd == NULL)
>>>>>>>  		return;
>>>>>>>  
>>>>>>> +	spin_lock(&kvm->mmu_lock);
>>>>>>>  	unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
>>>>>>> +	spin_unlock(&kvm->mmu_lock);
>>>>>>> +
>>>>>>
>>>>>> This ends up holding the spin lock for potentially quite a while, where
>>>>>> we can do things like __flush_dcache_area(), which I think can fault.
>>>>>
>>>>> I believe we're always using the linear mapping (or kmap on 32bit) in
>>>>> order not to fault.
>>>>>
>>>>
>>>> ok, then there's just the concern that we may be holding a spinlock for
>>>> a very long time.  I seem to recall Mario once added something where he
>>>> unlocked and gave a chance to schedule something else for each PUD or
>>>> something like that, because he ran into the issue during migration.  Am
>>>> I confusing this with something else?
>>>
>>> That definitely rings a bell: stage2_wp_range() uses that kind of trick
>>> to give the system a chance to breathe. Maybe we could use a similar
>>> trick in our S2 unmapping code? How about this (completely untested) patch:
>>>
>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>> index 962616fd4ddd..1786c24212d4 100644
>>> --- a/arch/arm/kvm/mmu.c
>>> +++ b/arch/arm/kvm/mmu.c
>>> @@ -292,8 +292,13 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
>>>  	phys_addr_t addr = start, end = start + size;
>>>  	phys_addr_t next;
>>>  
>>> +	BUG_ON(!spin_is_locked(&kvm->mmu_lock));

Nit: assert_spin_locked() is somewhat more pleasant (and currently looks
to expand to the exact same code).

Robin.

>>> +
>>>  	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
>>>  	do {
>>> +		if (need_resched() || spin_needbreak(&kvm->mmu_lock))
>>> +			cond_resched_lock(&kvm->mmu_lock);
>>> +
>>>  		next = stage2_pgd_addr_end(addr, end);
>>>  		if (!stage2_pgd_none(*pgd))
>>>  			unmap_stage2_puds(kvm, pgd, addr, next);
>>>
>>> The additional BUG_ON() is just for my own peace of mind - we seem to
>>> have missed a couple of these lately, and the "breathing" code makes
>>> it imperative that this lock is being taken prior to entering the
>>> function.
>>>
>>
>> Looks good to me!
> 
> OK. I'll stash that on top of Suzuki's series, and start running some
> actual tests... ;-)
> 
> Thanks,
> 
> 	M.
>