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

From: Marc Zyngier <[email-masked]>
Date: Thu, 21 May 2026 09:28:46 +0100

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?

Thanks,

	M.

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