linux-hardening - [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

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Date:   Tue, 27 Jul 2021 13:57:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-hardening@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

The it_present member of struct ieee80211_radiotap_header is treated as a
flexible array (multiple u32s can be conditionally present). In order for
memcpy() to reason (or really, not reason) about the size of operations
against this struct, use of bytes beyond it_present need to be treated
as part of the flexible array. Add a union/struct to contain the new
"bitmap" member, for use with trailing presence bitmaps and arguments.

Additionally improve readability in the iterator code which walks
through the bitmaps and arguments.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/net/ieee80211_radiotap.h | 24 ++++++++++++++++++++----
 net/mac80211/rx.c                |  2 +-
 net/wireless/radiotap.c          |  5 ++---
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/include/net/ieee80211_radiotap.h b/include/net/ieee80211_radiotap.h
index c0854933e24f..101c1e961032 100644
--- a/include/net/ieee80211_radiotap.h
+++ b/include/net/ieee80211_radiotap.h
@@ -39,10 +39,26 @@ struct ieee80211_radiotap_header {
 	 */
 	__le16 it_len;
 
-	/**
-	 * @it_present: (first) present word
-	 */
-	__le32 it_present;
+	union {
+		/**
+		 * @it_present: (first) present word
+		 */
+		__le32 it_present;
+
+		struct {
+			/* The compiler makes it difficult to overlap
+			 * a flex-array with an existing singleton,
+			 * so we're forced to add an empty named
+			 * variable here.
+			 */
+			struct { } __unused;
+
+			/**
+			 * @bitmap: all presence bitmaps
+			 */
+			__le32 bitmap[];
+		};
+	};
 } __packed;
 
 /* version is always 0 */
diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 2563473b5cf1..0d959a98e908 100644
--- a/net/mac80211/rx.c
+++ b/net/mac80211/rx.c
@@ -328,7 +328,7 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
 
 	rthdr = skb_push(skb, rtap_len);
 	memset(rthdr, 0, rtap_len - rtap.len - rtap.pad);
-	it_present = &rthdr->it_present;
+	it_present = rthdr->bitmap;
 
 	/* radiotap header, set always present flags */
 	rthdr->it_len = cpu_to_le16(rtap_len);
diff --git a/net/wireless/radiotap.c b/net/wireless/radiotap.c
index 36f1b59a78bf..9f4f1a772964 100644
--- a/net/wireless/radiotap.c
+++ b/net/wireless/radiotap.c
@@ -115,10 +115,9 @@ int ieee80211_radiotap_iterator_init(
 	iterator->_max_length = get_unaligned_le16(&radiotap_header->it_len);
 	iterator->_arg_index = 0;
 	iterator->_bitmap_shifter = get_unaligned_le32(&radiotap_header->it_present);
-	iterator->_arg = (uint8_t *)radiotap_header + sizeof(*radiotap_header);
+	iterator->_arg = (uint8_t *)&radiotap_header->bitmap[1];
 	iterator->_reset_on_ext = 0;
-	iterator->_next_bitmap = &radiotap_header->it_present;
-	iterator->_next_bitmap++;
+	iterator->_next_bitmap = &radiotap_header->bitmap[1];
 	iterator->_vns = vns;
 	iterator->current_namespace = &radiotap_ns;
 	iterator->is_radiotap_ns = 1;
-- 
2.30.2