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

[Marc Zyngier <maz@kernel.org>]

On Tue, 23 Sep 2025 10:18:59 +0100,
Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> On Mon, Sep 22, 2025 at 04:33:24PM -0700, Oliver Upton wrote:
> > On Mon, Sep 22, 2025 at 10:00:07PM +0100, Vincent Donnefort wrote:
> > > 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?
> > 
> > Yes, this is what I was alluding to on v1.
> > 
> > > 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.
> > 
> > I'm not sure I follow. Ranges are pretty commonly expressed as a range
> > terminated by an exclusive value. This just hasn't been an issue yet as
> > the page table code is only ever dealing with TTBR0 or VTTBR
> > translations.
> 
> If I do exclude the end boundary, evading checks would be as simple as making
> sure we overflow the end boundary?
> 
> e.g. __pkvm_host_share_guest(phys = 0xfffffffffffff000, size = 4096) 
> 
>         check_range_allowed_memory(phys, phys + size) /* nop */
> 	....
> 	for_each_hyp_page(page, phys, size) {  /* nop */
>                ...
> 	} 
> 	...
> 	/* Install a valid mapping to phys */
> 	kvm_pgtable_stage2_map(&vm->pgt, ipa, size, phys, ...) 

Why shouldn't this be as simple as this:

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 - 1);
}

which correctly deals with the boundary issue?

	M.

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