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

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Message-ID: <20170315091755.GL1277@cbox>
Date:   Wed, 15 Mar 2017 10:17:55 +0100
From:   Christoffer Dall <cdall@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, andreyknvl@...gle.com,
        dvyukov@...gle.com, marc.zyngier@....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 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?

Thanks,
-Christoffer