Re: [PATCH 1/3] kvm: arm/arm64: Take mmap_sem in stage2_unmap_vm

Paolo Bonzini
2017-03-15T13:29:07Z

On 15/03/2017 10:17, Christoffer Dall wrote:
> On Tue, Mar 14, 2017 at 02:52:32PM +0000, Suzuki K Poulose wrote:
>> From: Marc Zyngier <marc.zyngier@arm.com>
>>
>> We don't hold the mmap_sem while searching for the VMAs when
>> we try to unmap each memslot for a VM. Fix this properly to
>> avoid unexpected results.
>>
>> Fixes: commit 957db105c997 ("arm/arm64: KVM: Introduce stage2_unmap_vm")
>> Cc: stable@vger.kernel.org # v3.19+
>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>> arch/arm/kvm/mmu.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> index 962616f..f2e2e0c 100644
>> --- a/arch/arm/kvm/mmu.c
>> +++ b/arch/arm/kvm/mmu.c
>> @@ -803,6 +803,7 @@ void stage2_unmap_vm(struct kvm *kvm)
>> int idx;
>>
>> idx = srcu_read_lock(&kvm->srcu);
>> + down_read(&current->mm->mmap_sem);
>> spin_lock(&kvm->mmu_lock);
>>
>> slots = kvm_memslots(kvm);
>> @@ -810,6 +811,7 @@ void stage2_unmap_vm(struct kvm *kvm)
>> stage2_unmap_memslot(kvm, memslot);
>>
>> spin_unlock(&kvm->mmu_lock);
>> + up_read(&current->mm->mmap_sem);
>> srcu_read_unlock(&kvm->srcu, idx);
>> }
>>
>> --
>> 2.7.4
>>
>
> Are we sure that holding mmu_lock is valid while holding the mmap_sem?

Sure, spinlock-inside-semaphore and spinlock-inside-mutex is always okay.

Paolo