Re: [PATCH net] net: mctp: Don't access ifa_index when missing

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 05 May 2025 17:05:12 +0800 Matt Johnston wrote:
> +		/* Userspace programs providing AF_MCTP must be expecting ifa_index filter
> +		 * behaviour, as will those setting strict_check.
> +		 */
> +		if (hdr->ifa_family == AF_MCTP || cb->strict_check)
> +			ifindex = hdr->ifa_index;

The use of cb->strict_check is a bit strange here. I could be wrong but
I though cb->strict_check should only impact validation. Not be used
for changing behavior.

If you have a reason to believe all user space passes a valid header -
how about we just return an error if message is too short?
IPv4 and IPv6 seem to return an error if message is short and
cb->strict_check, so they are more strict. MCTP doesn't have a ton of
legacy user space, we don't have to be lenient at all. My intuition
would be to always act like IP acts under cb->strict_check