Re: [RFC PATCH] KVM: arm64: Fix double-free following kvm_pgtable_stage2_free_unlinked()

[Will Deacon]

On Tue, Feb 13, 2024 at 11:12:34AM +0000, Marc Zyngier wrote:
> On Mon, 12 Feb 2024 20:14:37 +0000,
> Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> > On Mon, Feb 12, 2024 at 07:30:52PM +0000, Will Deacon wrote:
> > > Sending this as an RFC as I only spotted it from code inspection and I'm
> > > surprised others aren't seeing fireworks if it's a genuine bug. I also
> > > couldn't come up with a sensible Fixes tag, as all of:
> > > 
> > >  e7c05540c694b ("KVM: arm64: Add helper for creating unlinked stage2 subtrees")
> > >  8f5a3eb7513fc ("KVM: arm64: Add kvm_pgtable_stage2_split()")
> > >  f6a27d6dc51b2 ("KVM: arm64: Drop last page ref in kvm_pgtable_stage2_free_removed()")
> 
> I'd blame it on the last commit, as we really ought to have it if we
> have the others.

Yes, that's probably the best approach if you're adding a Fixes tag.

> > > @@ -1502,7 +1501,6 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx,
> > >  
> > >  	if (!stage2_try_break_pte(ctx, mmu)) {
> > >  		kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level);
> > > -		mm_ops->put_page(childp);
> > >  		return -EAGAIN;
> > >  	}
> > 
> > This, on the other hand, seems possible. There exists a race where an
> > old block PTE could have the AF set on it and the underlying cmpxchg()
> > could fail. There shouldn't be a race with any software walkers, as we
> > hold the MMU lock for write here.
> 
> AF update is indeed a likely candidate.
> 
> In any case, this patch looks good to me as it is, and we can always
> have a separate tweak to adjust the severity of the first case as
> required. Unless anyone objects, I'd like to queue it shortly.

Fine by me! Even though I found it by inspection, I have taken the patch
for a spin to check that I (somehow) didn't break something else.

Will