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

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Oct 17, 2025 at 07:57:10AM +0000, Sebastian Ene wrote:
> Verify the offset to prevent OOB access in the hypervisor

I believe that would be just a read, so probably it would be difficult to use
this to compromise anything, except crashing the system?

> 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>
> ---
>  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 4e16f9b96f63..58b7d0c477d7 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -479,7 +479,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	struct ffa_mem_region_attributes *ep_mem_access;
>  	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 ||
> @@ -516,7 +516,12 @@ 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;
> +	}
> +
> +	if (fraglen < checked_offset) {
>  		ret = FFA_RET_INVALID_PARAMETERS;
>  		goto out_unlock;
>  

Perhaps this could be easier to reason about by moving this check with the nr_ranges?

        reg = (void *)buf + offset;
        if ((void *)reg->constituents > (void *)buf + fraglen) {
                ret = FFA_RET_INVALID_PARAMETERS;
                goto out_unlock;
        }
 
        nr_ranges = ((void *)buf + fraglen) - (void *)reg->constituents;
        if (nr_ranges % sizeof(reg->constituents[0])) {
                ret = FFA_RET_INVALID_PARAMETERS;

}
> -- 
> 2.51.0.858.gf9c4a03a3a-goog
>