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From: Johannes Berg <>
Cc: Kees Cook <>,
	Johannes Berg <>,, Sid Hayn <>
Subject: [PATCH] mac80211: fix radiotap header generation
Date: Tue,  9 Nov 2021 10:02:04 +0100	[thread overview]
Message-ID: <20211109100203.c61007433ed6.I1dade57aba7de9c4f48d68249adbae62636fd98c@changeid> (raw)

From: Johannes Berg <>

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.

Fixes: 8c89f7b3d3f2 ("mac80211: Use flex-array for radiotap header bitmap")
Reported-by: Sid Hayn <>
Signed-off-by: Johannes Berg <>
 net/mac80211/rx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index fc5c608d02e2..3562730ea0f8 100644
--- 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 */

             reply	other threads:[~2021-11-09  9:03 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-09  9:02 Johannes Berg [this message]
2021-11-09 16:13 ` [PATCH] mac80211: fix radiotap header generation Sid Hayn
2021-11-09 17:17 ` Kees Cook
  -- strict thread matches above, loose matches on Subject: below --
2009-10-28  8:58 Johannes Berg

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