linux-kernel - Re: [PATCH 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2

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Message-ID: <635795ed-7b12-bfb3-3b11-4edcfbc7b1bc@arm.com>
Date:   Wed, 15 Mar 2017 13:43:55 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Christoffer Dall <cdall@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, andreyknvl@...gle.com,
        dvyukov@...gle.com, christoffer.dall@...aro.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kcc@...gle.com,
        syzkaller@...glegroups.com, will.deacon@....com,
        catalin.marinas@....com, pbonzini@...hat.com, mark.rutland@....com,
        ard.biesheuvel@...aro.org, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd

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@...r.kernel.org # v3.10+
>>>>>> Cc: Marc Zyngier <marc.zyngier@....com>
>>>>>> Cc: Christoffer Dall <christoffer.dall@...aro.org>
>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>>>> ---
>>>>>>  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));
>> +
>>  	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.
-- 
Jazz is not dead. It just smells funny...