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

Marc Zyngier
2017-03-15T13:55:12Z

On 15/03/17 13:50, Robin Murphy wrote:
> 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@vger.kernel.org # v3.10+
>>>>>>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>>>>>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>>>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.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));
>
> Nit: assert_spin_locked() is somewhat more pleasant (and currently looks
> to expand to the exact same code).

Fancy!

Thanks,

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