[PATCH] mac80211: Use flex-array for radiotap header bitmap

[Kees Cook]

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 trailing flexible array and
initialize its initial index via pointer arithmetic.

Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: linux-wireless@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/net/ieee80211_radiotap.h | 4 ++++
 net/mac80211/rx.c                | 7 ++++++-
 net/wireless/radiotap.c          | 5 ++---
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/net/ieee80211_radiotap.h b/include/net/ieee80211_radiotap.h
index c0854933e24f..6b7274edb3c6 100644
--- a/include/net/ieee80211_radiotap.h
+++ b/include/net/ieee80211_radiotap.h
@@ -43,6 +43,10 @@ struct ieee80211_radiotap_header {
 	 * @it_present: (first) present word
 	 */
 	__le32 it_present;
+	/**
+	 * @it_optional: all remaining presence bitmaps
+	 */
+	__le32 it_optional[];
 } __packed;
 
 /* version is always 0 */
diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 3eb7b03b23c6..33c56eab07fc 100644
--- a/net/mac80211/rx.c
+++ b/net/mac80211/rx.c
@@ -359,7 +359,12 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
 
 	put_unaligned_le32(it_present_val, it_present);
 
-	pos = (void *)(it_present + 1);
+	/* This references through an offset into it_optional[] rather
+	 * than via it_present otherwise later uses of pos will cause
+	 * the compiler to think we have walked past the end of the
+	 * struct member.
+	 */
+	pos = (void *)&rthdr->it_optional[it_present - rthdr->it_optional];
 
 	/* the order of the following fields is important */
 
diff --git a/net/wireless/radiotap.c b/net/wireless/radiotap.c
index 8099c9564a59..ae2e1a896461 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->it_optional;
 	iterator->_reset_on_ext = 0;
-	iterator->_next_bitmap = &radiotap_header->it_present;
-	iterator->_next_bitmap++;
+	iterator->_next_bitmap = radiotap_header->it_optional;
 	iterator->_vns = vns;
 	iterator->current_namespace = &radiotap_ns;
 	iterator->is_radiotap_ns = 1;
-- 
2.30.2