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 18:51:26 +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 18:43, Christoffer Dall wrote:
> On Mon, May 15, 2017 at 02:36:58PM +0100, Suzuki K Poulose wrote:
>> On 15/05/17 11:00, Christoffer Dall wrote:
>>> Hi Suzuki,
>>> 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.
>>
> Yes, absolutely.  I've already applied patch 1 so no need to include
> that in your respin.

I have made a minor change to the 1st patch, to make use of READ_ONCE/WRITE_ONCE,
to make sure we don't use the cached value of the kvm->arch.pgd. Something like :


@@ -829,22 +829,22 @@ void stage2_unmap_vm(struct kvm *kvm)
   * Walks the level-1 page table pointed to by kvm->arch.pgd and frees all
   * underlying level-2 and level-3 tables before freeing the actual level-1 table
   * and setting the struct pointer to NULL.
- *
- * Note we don't need locking here as this is only called when the VM is
- * destroyed, which can only be done once.
   */
  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 = READ_ONCE(kvm->arch.pgd);
+		WRITE_ONCE(kvm->arch.pgd, NULL);
+	}
         spin_unlock(&kvm->mmu_lock);
  

Let me know if you could fix it up or I could send a fresh series. Sorry about that.

Suzuki