Re: [PATCH v6 4/6] KVM: arm64: Fix bounds checking in do_ffa_mem_reclaim()

[Sebastian Ene <sebastianene@google.com>]

On Wed, May 27, 2026 at 03:02:34PM +0000, Mostafa Saleh 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.
> 
> Also, drop the WARN_ONs as that will panic the kernel and in the
> next checks there are no WARNs, so that makes it consistent.
> 
> Fixes: 0a9f15fd5674 ("KVM: arm64: pkvm: Add support for fragmented FF-A descriptors")
> Signed-off-by: Mostafa Saleh <smostafa@google.com>
> ---
>  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 1af722771178..b6cf9ad82e12 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -607,8 +607,8 @@ 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 ||
> -		    fraglen > KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE)) {
> +	if (offset + CONSTITUENTS_OFFSET(0) > len ||
> +	    fraglen > KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE) {
>  		ret = FFA_RET_ABORTED;
>  		ffa_rx_release(res);
>  		goto out_unlock;
> @@ -641,6 +641,11 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
>  		goto out_unlock;
>  
>  	reg = (void *)buf + offset;
> +	if (offset + CONSTITUENTS_OFFSET(reg->addr_range_cnt) > len) {
> +		ret = FFA_RET_ABORTED;
> +		goto out_unlock;
> +	}
> +

I wonder if we should do this check first and then mem_reclaim, the rational being that if the check fails
the page will be lost forever.

```
	reg = (void *)buf + offset;                                                                                                                                                                            
	if (offset + CONSTITUENTS_OFFSET(reg->addr_range_cnt) > len) {                                                                                                                                         
		ret = FFA_RET_ABORTED;                                                                                                                                                                         
		goto out_unlock;                                                                                                                                                                               
	}

	ffa_mem_reclaim(res, handle_lo, handle_hi, flags);                                                                                                                                                    
	if (res->a0 != FFA_SUCCESS)                                                                                                                                                                           
		goto out_unlock; 
```                                                                                                                                                                                                      

>  	/* If the SPMD was happy, then we should be too. */
>  	WARN_ON(ffa_host_unshare_ranges(reg->constituents,
>  					reg->addr_range_cnt));
> -- 
> 2.54.0.746.g67dd491aae-goog
>

Sebastian