Re: [RFC PATCH 1/3] KVM: arm64: Fix possible memory leak in kvm stage2

[wangyanan (Y)]

Hi Will,

On 2020/11/30 21:21, Will Deacon wrote:
> On Mon, Nov 30, 2020 at 08:18:45PM +0800, Yanan Wang wrote:
> > When installing a new leaf pte onto an invalid ptep, we need to get_page(ptep).
> > When just updating a valid leaf ptep, we shouldn't get_page(ptep).
> > Incorrect page_count of translation tables might lead to memory leak,
> > when unmapping a stage 2 memory range.
> Did you find this by inspection, or did you hit this in practice? I'd be
> interested to see the backtrace for mapping over an existing mapping.

Actually this is found by inspection.

In the current code, get_page() will uniformly called at "out_get_page" 
in function stage2_map_walk_leaf(),

no matter the old ptep is valid or not.

When using stage2_unmap_walker() API to unmap a memory range, some 
page-table pages might not be

freed if page_count of the pages is not right.

> > Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> > ---
> >   arch/arm64/kvm/hyp/pgtable.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 0271b4a3b9fe..696b6aa83faf 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -186,6 +186,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
> >   		return old == pte;
> >   
> >   	smp_store_release(ptep, pte);
> > +	get_page(virt_to_page(ptep));
> This is also used for the hypervisor stage-1 page-table, so I'd prefer to
> leave this function as-is.
I agree at this point.
> >   	return true;
> >   }
> >   
> > @@ -476,6 +477,7 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> >   	/* There's an existing valid leaf entry, so perform break-before-make */
> >   	kvm_set_invalid_pte(ptep);
> >   	kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> > +	put_page(virt_to_page(ptep));
> >   	kvm_set_valid_leaf_pte(ptep, phys, data->attr, level);
> >   out:
> >   	data->phys += granule;
> Isn't this hunk alone sufficient to solve the problem?
>
> Will
> .

Not sufficient enough. When the old ptep is valid and old pte equlas new 
pte, in this case, "True" is also returned by kvm_set_valid_leaf_pte()

and get_page() will still be called.


Yanan