Subject: [PATCH 2/2] kvm: arm/arm64: Fix use after free of stage2 page table

From: [email-masked] (Christoffer Dall)
Date: Mon, 15 May 2017 20:22:06 +0200

On Mon, May 15, 2017 at 06:51:26PM +0100, Suzuki K Poulose wrote:
> 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);

I'm not sure I understand this.  What do you use pgd for?  Wouldn't it
be cleaner t use the READ_ONCE() in the if statement?

> +		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.
> 

At this point it may be better to send a new series with two patches
against what I already have in kvmarm/master.

> Sorry about that.

No worries at all.

Thanks,
-Christoffer