Re: [PATCH 1/1] net: netlink: Fix multicast group storage allocation for families with more than one groups

[David Miller]

From: Matti Vaittinen <matti.vaittinen@xxxxxxxxx>
Date: Mon, 11 Jan 2016 14:26:19 +0200

> Multicast groups are stored in global buffer. Check for needed buffer size
> incorrectly compares buffer size to first id for family. This means that
> for families with more than one mcast id one may allocate too small buffer
> and end up writing rest of the groups to some unallocated memory. Fix the 
> buffer size check to compare allocated space to last mcast id for the 
> family.
> 
> Tested on ARM using kernel 3.14
> 
> Signed-off-by: Matti Vaittinen <matti.vaittinen@xxxxxxxxx>

Indeed, it looks like this function was never tested with any value
of n_groups other than one.

But I think your change has an off-by-one bug:

> -		if (id >= mc_groups_longs * BITS_PER_LONG) {
> +		if (id + n_groups >= mc_groups_longs * BITS_PER_LONG) {

I think this needs to be "id + n_groups > ".  Consider the existing,
working, case of "n_groups == 1".  Now you're adding '1' and therefore
the test needs to be adjusted from >= to >.

Thanks.