mac80211: fix radiotap header generation

Origin: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=c033a38a81bc539d6c0db8c5387e0b14d819a0cf

In commit 8c89f7b3d3f2 ("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: 8c89f7b3d3f2 ("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>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name mac80211-fix-radiotap-header-generation.patch

diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index c4071b015c18848ba5cbbc019d12890b3ad2640f..419f06ef8c986afd30b2f6014018eff5de7c3bfb 100644 (file)
--- a/net/mac80211/rx.c
+++ b/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 */