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

[Oliver Upton]

On Mon, Feb 12, 2024 at 08:14:41PM +0000, Oliver Upton wrote:
> On Mon, Feb 12, 2024 at 07:30:52PM +0000, Will Deacon wrote:
> > kvm_pgtable_stage2_free_unlinked() does the final put_page() on the
> > root page of the sub-tree before returning, so remove the additional
> > put_page() invocations in the callers.
> > 
> > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> > Cc: Ricardo Koller <ricarkol@xxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> > 
> > Hi folks,
> > 
> > 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()")
> > 
> > are actually ok in isolation. Hrm. Please tell me I'm wrong?
> > 
> >  arch/arm64/kvm/hyp/pgtable.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index c651df904fe3..ab9d05fcf98b 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1419,7 +1419,6 @@ kvm_pte_t *kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> >  				 level + 1);
> >  	if (ret) {
> >  		kvm_pgtable_stage2_free_unlinked(mm_ops, pgtable, level);
> > -		mm_ops->put_page(pgtable);
> >  		return ERR_PTR(ret);
> >  	}
> 
> AFAICT, this entire branch is effectively dead code, unless there's a
> KVM bug lurking behind the page table walk. The sub-tree isn't visible
> to other software or hardware walkers yet, so none of the PTE races
> could cause this to pop.
> 
> So while this is very obviously a bug, it might be pure luck that folks
> haven't seen smoke here. Perhaps while fixing the bug we should take the
> opportunity to promote the condition to WARN_ON_ONCE().
> 
> > @@ -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.

That is to say, a race with a *HW* update to the PTE that sets AF.

> There shouldn't be a race with any software walkers, as we
> hold the MMU lock for write here.
> 
> -- 
> Thanks,
> Oliver

-- 
Thanks,
Oliver