linux-kernel - Re: [PATCH 3.16 121/178] kvm: arm/arm64: Fix locking for kvm_free_stage2

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Message-ID: <952e43a7-0073-b068-5ca8-03ec6bb8a363@arm.com>
Date:   Mon, 17 Jul 2017 16:12:08 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     akpm@...ux-foundation.org, Mark Rutland <mark.rutland@....com>,
        Paolo Bonzini <pbonzin@...hat.com>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH 3.16 121/178] kvm: arm/arm64: Fix locking for
 kvm_free_stage2_pgd

On 16/07/17 14:56, Ben Hutchings wrote:
> 3.16.46-rc1 review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Suzuki K Poulose <suzuki.poulose@....com>
>
> commit 8b3405e345b5a098101b0c31b264c812bba045d9 upstream.
>
> 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. And since we have to unmap the entire Guest memory range
> holding a spinlock, make sure we yield the lock if necessary, after we
> unmap each PUD range.
>
> Fixes: commit d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
> Cc: Paolo Bonzini <pbonzin@...hat.com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Christoffer Dall <christoffer.dall@...aro.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> [ Avoid vCPU starvation and lockup detector warnings ]
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Christoffer Dall <cdall@...aro.org>
> [bwh: Backported to 3.16:
>  - unmap_stage2_range() is a wrapper around unmap_range(), which is also used for
>    HYP page table setup.  So unmap_range() should do the cond_resched_lock(), but
>    only if kvm != NULL.
>  - Adjust context]
> Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
> ---
>  arch/arm/kvm/mmu.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -199,6 +199,12 @@ static void unmap_range(struct kvm *kvm,
>  		next = kvm_pgd_addr_end(addr, end);
>  		if (!pgd_none(*pgd))
>  			unmap_puds(kvm, pgd, addr, next);
> +		/*
> +		 * If the range is too large, release the kvm->mmu_lock
> +		 * to prevent starvation and lockup detector warnings.
> +		 */
> +		if (kvm && next != end)
> +			cond_resched_lock(&kvm->mmu_lock);
>  	} while (pgd++, addr = next, addr != end);
>  }
>
> @@ -553,6 +559,7 @@ int kvm_alloc_stage2_pgd(struct kvm *kvm
>   */
>  static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
>  {
> +	assert_spin_locked(&kvm->mmu_lock);
>  	unmap_range(kvm, kvm->arch.pgd, start, size);
>  }
>
> @@ -637,7 +644,10 @@ 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);
> +
>  	free_pages((unsigned long)kvm->arch.pgd, S2_PGD_ORDER);
>  	kvm->arch.pgd = NULL;
>  }

Hi Ben,

There is a follow up patch for this one to make sure we check/free the PGD under the
mmu_lock.

commit 6c0d706b563af73 ("kvm: arm/arm64: Fix race in resetting stage2 PGD")

So unless you want to fold that in, this looks alright. Looks like we missed a Cc:
stable for that. I will send it to stable soon.

Cheers
Suzuki



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