Re: [PATCH v7 6/7] KVM: arm64: Ensure FFA ranges are page aligned

[Sebastian Ene <sebastianene@google.com>]

On Thu, Jun 18, 2026 at 06:09:11PM +0100, Vincent Donnefort wrote:
> On Wed, Jun 17, 2026 at 02:51:29PM +0000, Sebastian Ene wrote:
> > From: Mostafa Saleh <smostafa@google.com>
> > 
> > At the moment we only check that the size of the range is page
> > aligned, and truncate the address to the page boundary.
> > This make an assumption that TZ will do the same.
> > 
> > However, it might decide to use the extra offset of the neighbour
> > page at the end, which is valid under FFA if NS is using larger
> > page size.
> 

Hey Vincent,

> I failed to parse this 
> 
> But I see 
> 
> /* The base IPA of the constituent memory region, aligned to 4 kiB */ 
> 
> So it sounds fair to prevent oversharing when PAGE_SIZE > 4KiB
> 

I think the problem is when you have a mismatch between FFA_PAGE_SIZE
and the system PAGE_SIZE. We expect a fixed FFA_PAGE_SIZE of 4kb and
this is enforced by :
https://elixir.bootlin.com/linux/v7.1.2/source/arch/arm64/kvm/hyp/nvhe/ffa.c#L761

if FFA_PAGE_SIZE = 4kb and PAGE_SIZE = 16kb you can end up annotating more pages with FF-A then
needed when the range->address is unaligned.

It took me a while to understand this so I guess it is better to rephrase the commit msg.

> > 
> > Harden this check by also checking that the base address is aligned
> > and reject it otherwise.
> > 
> > Fixes: 436090001776 ("KVM: arm64: Handle FFA_MEM_SHARE calls from the host")
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > Signed-off-by: Sebastian Ene <sebastianene@google.com>
> 
> Perhaps the commit description needs some improvement.
> 
> The rest looks good.
> 
> Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
> 
> > ---
> >  arch/arm64/kvm/hyp/nvhe/ffa.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > index 1a2abd0154c6..d7c5701d0584 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > @@ -352,7 +352,7 @@ static u32 __ffa_host_share_ranges(struct ffa_mem_region_addr_range *ranges,
> >  		u64 sz = (u64)range->pg_cnt * FFA_PAGE_SIZE;
> >  		u64 pfn = hyp_phys_to_pfn(range->address);
> >  
> > -		if (!PAGE_ALIGNED(sz))
> > +		if (!PAGE_ALIGNED(sz | range->address))
> >  			break;
> >  
> >  		if (__pkvm_host_share_ffa(pfn, sz / PAGE_SIZE))
> > @@ -372,7 +372,7 @@ static u32 __ffa_host_unshare_ranges(struct ffa_mem_region_addr_range *ranges,
> >  		u64 sz = (u64)range->pg_cnt * FFA_PAGE_SIZE;
> >  		u64 pfn = hyp_phys_to_pfn(range->address);
> >  
> > -		if (!PAGE_ALIGNED(sz))
> > +		if (!PAGE_ALIGNED(sz | range->address))
> >  			break;
> >  
> >  		if (__pkvm_host_unshare_ffa(pfn, sz / PAGE_SIZE))
> > -- 
> > 2.54.0.1136.gdb2ca164c4-goog
> > 

Thanks,
Sebastian