mac80211: fix radiotap header generation

In commit 8c89f7b ("mac80211: Use flex-array for radiotap header
bitmap") we accidentally pointed the position to the wrong place, so
we overwrite a present bitmap, and thus cause all kinds of trouble.

To see the issue, note that the previous code read:

  pos = (void *)(it_present + 1);

The requirement now is that we need to calculate pos via it_optional,
to not trigger the compiler hardening checks, as:

  pos = (void *)&rthdr->it_optional[...];

Rewriting the original expression, we get (obviously, since that just
adds "+ x - x" terms):

  pos = (void *)(it_present + 1 + rthdr->it_optional - rthdr->it_optional)

and moving the "+ rthdr->it_optional" outside to be used as an array:

  pos = (void *)&rthdr->it_optional[it_present + 1 - rthdr->it_optional];

The original is off by one, fix it.

Cc: stable@vger.kernel.org
Fixes: 8c89f7b ("mac80211: Use flex-array for radiotap header bitmap")
Reported-by: Sid Hayn <sidhayn@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Sid Hayn <sidhayn@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211109100203.c61007433ed6.I1dade57aba7de9c4f48d68249adbae62636fd98c@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -364,7 +364,7 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
 	 * the compiler to think we have walked past the end of the
 	 * struct member.
 	 */
-	pos = (void *)&rthdr->it_optional[it_present - rthdr->it_optional];
+	pos = (void *)&rthdr->it_optional[it_present + 1 - rthdr->it_optional];
 
 	/* the order of the following fields is important */