[PATCH 39/64] mac80211: Use memset_after() to clear tx status

[Johannes Berg]

On Sat, 2021-07-31 at 08:55 -0700, Kees Cook wrote:
> On Tue, Jul 27, 2021 at 01:58:30PM -0700, Kees Cook wrote:
> > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > field bounds checking for memset(), avoid intentionally writing across
> > neighboring fields.
> > 
> > Use memset_after() so memset() doesn't get confused about writing
> > beyond the destination member that is intended to be the starting point
> > of zeroing through the end of the struct.
> > 
> > Note that the common helper, ieee80211_tx_info_clear_status(), does NOT
> > clear ack_signal, but the open-coded versions do. All three perform
> > checks that the ack_signal position hasn't changed, though.
> 
> Quick ping on this question: there is a mismatch between the common
> helper and the other places that do this. Is there a bug here?

Yes.

The common helper should also clear ack_signal, but that was broken by
commit e3e1a0bcb3f1 ("mac80211: reduce IEEE80211_TX_MAX_RATES"), because
that commit changed the order of the fields and updated carl9170 and p54
properly but not the common helper...

It doesn't actually matter much because ack_signal is normally filled in
afterwards, and even if it isn't, it's just for statistics.

The correct thing to do here would be to

	memset_after(&info->status, 0, rates);

johannes