Re: [PATCH 6.6.y] KVM: arm64: Check the untrusted offset in FF-A memory share

[Sebastian Ene <sebastianene@google.com>]

On Mon, Nov 24, 2025 at 03:50:45PM +0100, Greg KH wrote:
> On Mon, Nov 24, 2025 at 09:11:34AM -0500, Sasha Levin wrote:
> > From: Sebastian Ene <sebastianene@google.com>
> > 
> > [ Upstream commit 103e17aac09cdd358133f9e00998b75d6c1f1518 ]
> > 
> > Verify the offset to prevent OOB access in the hypervisor
> > FF-A buffer in case an untrusted large enough value
> > [U32_MAX - sizeof(struct ffa_composite_mem_region) + 1, U32_MAX]
> > is set from the host kernel.
> > 
> > Signed-off-by: Sebastian Ene <sebastianene@google.com>
> > Acked-by: Will Deacon <will@kernel.org>
> > Link: https://patch.msgid.link/20251017075710.2605118-1-sebastianene@google.com
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/ffa.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > index 8d21ab904f1a9..eacf4ba1d88e9 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > @@ -425,7 +425,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
> >  	DECLARE_REG(u32, npages_mbz, ctxt, 4);
> >  	struct ffa_composite_mem_region *reg;
> >  	struct ffa_mem_region *buf;
> > -	u32 offset, nr_ranges;
> > +	u32 offset, nr_ranges, checked_offset;
> >  	int ret = 0;
> >  
> >  	if (addr_mbz || npages_mbz || fraglen > len ||
> > @@ -460,7 +460,12 @@ static void __do_ffa_mem_xfer(const u64 func_id,
> >  		goto out_unlock;
> >  	}
> >  
> > -	if (fraglen < offset + sizeof(struct ffa_composite_mem_region)) {
> > +	if (check_add_overflow(offset, sizeof(struct ffa_composite_mem_region), &checked_offset)) {
> > +		ret = FFA_RET_INVALID_PARAMETERS;
> > +		goto out_unlock;
> > +	}

hello Greg,

> 
> I was told that a "straight" backport like this was not correct, so we
> need a "better" one :(
> 
> Sebastian, can you provide the correct backport for 6.6.y please?
> 

I think Sasha's patch is doing the right thing. Sasha thanks for
posting it so fast.

I looked up the other faild patches on stable and the reason why the patch doesn't
apply is because we don't have the FF-A proxy inthe following versions:
 - 5.4, 5.10, 5.15, 6.1

> thanks,
> 
> greg k-h

thanks,
Sebastian