linux-kernel - Re: [PATCH 2/2] kvm: arm/arm64: Fix race in resetting stage2 PGD

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Date:   Mon, 24 Apr 2017 14:27:01 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     pbonzini@...hat.com, christoffer.dall@...aro.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        marc.zyngier@....com, mark.rutland@....com, andreyknvl@...gle.com,
        rkrcmar@...hat.com
Subject: Re: [PATCH 2/2] kvm: arm/arm64: Fix race in resetting stage2 PGD

On Mon, Apr 24, 2017 at 11:10:24AM +0100, Suzuki K Poulose wrote:
> In kvm_free_stage2_pgd() we check the stage2 PGD before holding
> the lock and proceed to take the lock if it is valid. And we unmap
> the page tables, followed by releasing the lock. We reset the PGD
> only after dropping this lock, which could cause a race condition
> where another thread waiting on the lock could potentially see that
> the PGD is still valid and proceed to perform a stage2 operation.
> 
> This patch moves the stage2 PGD manipulation under the lock.
> 
> Reported-by: Alexander Graf <agraf@...e.de>
> Cc: Christoffer Dall <christoffer.dall@...aro.org>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>

Reviewed-by: Christoffer Dall <cdall@...aro.org>

> ---
>  arch/arm/kvm/mmu.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 582a972..9c4026d 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -835,16 +835,18 @@ void stage2_unmap_vm(struct kvm *kvm)
>   */
>  void kvm_free_stage2_pgd(struct kvm *kvm)
>  {
> -	if (kvm->arch.pgd == NULL)
> -		return;
> +	void *pgd = NULL;
>  
>  	spin_lock(&kvm->mmu_lock);
> -	unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
> +	if (kvm->arch.pgd) {
> +		unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
> +		pgd = kvm->arch.pgd;
> +		kvm->arch.pgd = NULL;
> +	}
>  	spin_unlock(&kvm->mmu_lock);
> -
>  	/* Free the HW pgd, one page at a time */
> -	free_pages_exact(kvm->arch.pgd, S2_PGD_SIZE);
> -	kvm->arch.pgd = NULL;
> +	if (pgd)
> +		free_pages_exact(pgd, S2_PGD_SIZE);
>  }
>  
>  static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
> -- 
> 2.7.4
>