linux-kernel - Re: [PATCH 2/2] kvm: arm/arm64: Fix use after free of stage2 page table

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Date:   Mon, 15 May 2017 14:36:58 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Christoffer Dall <cdall@...aro.org>
Cc:     christoffer.dall@...aro.org, agraf@...e.de, andreyknvl@...gle.com,
        marc.zyngier@....com, mark.rutland@....com, pbonzini@...hat.com,
        rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] kvm: arm/arm64: Fix use after free of stage2 page
 table

On 15/05/17 11:00, Christoffer Dall wrote:
> Hi Suzuki,
>
> On Wed, May 03, 2017 at 03:17:52PM +0100, Suzuki K Poulose wrote:
>> We yield the kvm->mmu_lock occassionaly while performing an operation
>> (e.g, unmap or permission changes) on a large area of stage2 mappings.
>> However this could possibly cause another thread to clear and free up
>> the stage2 page tables while we were waiting for regaining the lock and
>> thus the original thread could end up in accessing memory that was
>> freed. This patch fixes the problem by making sure that the stage2
>> pagetable is still valid after we regain the lock. The fact that
>> mmu_notifer->release() could be called twice (via __mmu_notifier_release
>> and mmu_notifier_unregsister) enhances the possibility of hitting
>> this race where there are two threads trying to unmap the entire guest
>> shadow pages.
>>
>> While at it, cleanup the redudant checks around cond_resched_lock in
>> stage2_wp_range(), as cond_resched_lock already does the same checks.
>>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: andreyknvl@...gle.com
>> 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>
>> ---
>>  arch/arm/kvm/mmu.c | 17 ++++++++++++-----
>>  1 file changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> index 909a1a7..5b3e0db 100644
>> --- a/arch/arm/kvm/mmu.c
>> +++ b/arch/arm/kvm/mmu.c
>> @@ -301,9 +301,14 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
>>  		/*
>>  		 * If the range is too large, release the kvm->mmu_lock
>>  		 * to prevent starvation and lockup detector warnings.
>> +		 * Make sure the page table is still active when we regain
>> +		 * the lock.
>>  		 */
>> -		if (next != end)
>> +		if (next != end) {
>>  			cond_resched_lock(&kvm->mmu_lock);
>> +			if (!READ_ONCE(kvm->arch.pgd))
>> +				break;
>> +		}
>
> So I don't think this change is wrong, but I wonder if it's sufficient.
> For example, I can see that this function is called from
>
> stage2_unmsp_vm
>  -> stage2_unmap_memslot
>    -> unmap_stage2_range
>
> and
>
> kvm_arch_flush_shadow_memslot
>  -> unmap_stage2_range
>
> which never check if the pgd pointer is valid,

You are right. Those two callers do not check it. We could fix all of this by simply
moving the check to the beginning of the loop.
i.e, something like this :

@@ -295,6 +295,12 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
	assert_spin_locked(&kvm->mmu_lock);
	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
	do {
+		/*
+		 * Make sure the page table is still active, as we could
+		 * another thread could have possibly freed the page table.
+		 */
+		if (!READ_ONCE(kvm->arch.pgd))
+			break;
		next = stage2_pgd_addr_end(addr, end);
		if (!stage2_pgd_none(*pgd))
			unmap_stage2_puds(kvm, pgd, addr, next);




> and finally, kvm_free_stage2_pgd also checks the pgd pointer outside of holding the
> kvm->mmu_lock so why is this not racy?

This has been fixed by patch 1 in the series. So should be fine.


I can respin the patch with the changes if you are OK with it.

Suzuki