linux-kernel - Re: [PATCH 1/3] kvm: arm/arm64: Take mmap_sem in stage2_unmap

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Date:   Wed, 15 Mar 2017 09:34:53 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Christoffer Dall <cdall@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>
Cc:     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 1/3] kvm: arm/arm64: Take mmap_sem in stage2_unmap_vm

On 15/03/17 09:17, Christoffer Dall wrote:
> On Tue, Mar 14, 2017 at 02:52:32PM +0000, Suzuki K Poulose wrote:
>> From: Marc Zyngier <marc.zyngier@....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@...r.kernel.org # v3.19+
>> Cc: Christoffer Dall <christoffer.dall@...aro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....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?

Maybe I'm just confused by the many levels of locking, Here's my rational:

- kvm->srcu protects the memslot list
- mmap_sem protects the kernel VMA list
- mmu_lock protects the stage2 page tables (at least here)

I don't immediately see any issue with holding the mmap_sem mutex here
(unless there is a path that would retrigger a down operation on the
mmap_sem?).

Or am I missing something obvious?

Thanks,

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