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

[Marc Zyngier <maz@kernel.org>]

On Tue, 13 Feb 2024 16:29:42 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> 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@linux.dev> 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@kernel.org>
> > > > Cc: Oliver Upton <oliver.upton@linux.dev>
> > > > Cc: Ricardo Koller <ricarkol@google.com>
> > > > Signed-off-by: Will Deacon <will@kernel.org>
> > > > ---
> > > > 
> > > > 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()")
> > 
> > I'd blame it on the last commit, as we really ought to have it if we
> > have the others.
> > 
> > > >
> > > > 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().
> > 
> > Can't you construct a case where an allocation fails during the walk
> > (memcache empty), and we end up on this exact path?
> 
> Possibly, but AFAICT that can only happen if there was a bug in KVM. We
> don't start the walk at all if userspace set the split chunk size to 0,
> and otherwise we expect it to be at least PMD_SIZE, which will top up
> the cache to 1 every pass. stage2_split_walker() will 'do the right
> thing' if there aren't enough preallocated pages to get down to level 3.
> 
> It really doesn't matter either way, I'm just trying to convince myself
> of the reasons why we haven't seen this explode yet :)

Yeah, that's probably very unlikely to hit given the current
conditions.

> 
> > > 
> > > > @@ -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.
> 
> Agreed, happy with the way this looks and should've added:
> 
> Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
> 
> the first time around.

Thanks for that. I'll queue that shortly and send (another) PR.

	M.

-- 
Without deviation from the norm, progress is not possible.

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