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

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Date:   Wed, 15 Mar 2017 10:21:47 +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 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd

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.

Is that valid?

Thanks,
-Christoffer

>  	/* Free the HW pgd, one page at a time */
>  	free_pages_exact(kvm->arch.pgd, S2_PGD_SIZE);
>  	kvm->arch.pgd = NULL;
> -- 
> 2.7.4
>