Re: [PATCH v2] KVM: arm64: Check range args for pKVM mem transitions

[Vincent Donnefort <vdonnefort@google.com>]

On Sun, Sep 21, 2025 at 12:29:08PM +0100, Marc Zyngier wrote:
> On Fri, 19 Sep 2025 16:50:56 +0100,
> Vincent Donnefort <vdonnefort@google.com> wrote:
> > 
> > There's currently no verification for host issued ranges in most of the
> > pKVM memory transitions. The subsequent end boundary might therefore be
> > subject to overflow and could evade the later checks.
> > 
> > Close this loophole with an additional check_range_args() check on a per
> > public function basis.
> > 
> > host_unshare_guest transition is already protected via
> > __check_host_shared_guest(), while assert_host_shared_guest() callers
> > are already ignoring host checks.
> > 
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > 
> > ---
> > 
> >  v1 -> v2:
> >    - Also check for (nr_pages * PAGE_SIZE) overflow. (Quentin)
> >    - Rename to check_range_args().
> > 
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index 8957734d6183..65fcd2148f59 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -712,6 +712,14 @@ static int __guest_check_page_state_range(struct pkvm_hyp_vm *vm, u64 addr,
> >  	return check_page_state_range(&vm->pgt, addr, size, &d);
> >  }
> >  
> > +static bool check_range_args(u64 start, u64 nr_pages, u64 *size)
> > +{
> > +	if (check_mul_overflow(nr_pages, PAGE_SIZE, size))
> > +		return false;
> > +
> > +	return start < (start + *size);
> 
> I will echo Oliver's concern on v1: you probably want to convert the
> boundary check to be inclusive of the end of the range. Otherwise, a
> range that ends at the top of the 64bit range will be represented as
> 0, and fail the  check despite being perfectly valid.

Do you mean allowing something like start == 0xfffffffffffff000 and size ==
4096?

But I guess that would still put all the following checks using "addr + size" at
risk. Also, I believe even the code in pgtable.c wouldn't support a such range
as it is also using a u64 end boundary.

> 
> That's not a problem for PAs, as we will be stuck with at most 56bit
> PAs for quite a while, but VAs are a different story, and this sort of
> range check should be valid for VAs as well.
> 
> Thanks,
> 
> 	M.
> 
> -- 
> Jazz isn't dead. It just smells funny.