Subject: Re: [PATCH v4 4/5] KVM: arm64: Fix bounds checking in do_ffa_mem_reclaim()

From: Mostafa Saleh <[email-masked]>
Date: Thu, 21 May 2026 10:30:03 +0000

Hi Marc,

On Thu, May 21, 2026 at 09:28:46AM +0100, Marc Zyngier wrote:
> On Wed, 20 May 2026 21:49:47 +0100,
> Mostafa Saleh <[email-masked]> wrote:
> > 
> > Sashiko (locally) reports out of bound write possiblity if SPMD
> > returns an invalid data.
> > 
> > While SPMD is considered trusted, pKVM does some basic checks,
> > for offset to be less than or equal len.
> > 
> > However, that is incorrect as even if the offset is smaller than
> > len pKVM can still access out of bound memory in the next
> > ffa_host_unshare_ranges().
> > 
> > Split this check into 2:
> > 1- Check that the fixed portion of the descriptor fits.
> > 2- After getting reg, check the variable array size addr_range_cnt
> >    fits.
> > 
> > Signed-off-by: Mostafa Saleh <[email-masked]>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/ffa.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > index 1af722771178..e6aa2bfa63b1 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > @@ -607,7 +607,7 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
> >  	 * check that we end up with something that doesn't look _completely_
> >  	 * bogus.
> >  	 */
> > -	if (WARN_ON(offset > len ||
> > +	if (WARN_ON(offset + CONSTITUENTS_OFFSET(0) > len ||
> >  		    fraglen > KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE)) {
> 
> Do you really want to keep this a WARN_ON(), given that this results
> in a panic in most pKVM configurations?

Which kind of configuration will that check fail on?
Does that mean at the moment pKVM does out-of-bound access for
the header?

Thanks,
Mostafa

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