[PATCH v2] mac80211: Fix TKIP replay protection immediately after key setup

[PATCH v2] mac80211: Fix TKIP replay protection immediately after key setup

[Jouni Malinen]

TKIP replay protection was skipped for the very first frame received
after a new key is configured. While this is potentially needed to avoid
dropping a frame in some cases, this does leave a window for replay
attacks with group-addressed frames at the station side. Any earlier
frame sent by the AP using the same key would be accepted as a valid
frame and the internal RSC would then be updated to the TSC from that
frame. This would allow multiple previously transmitted group-addressed
frames to be replayed until the next valid new group-addressed frame
from the AP is received by the station.

Fix this by limiting the no-replay-protection exception to apply only
for the case where TSC=0, i.e., when this is for the very first frame
protected using the new key, and the local RSC had not been set to a
higher value when configuring the key (which may happen with GTK).

Signed-off-by: Jouni Malinen <j@w1.fi>
---
 net/mac80211/tkip.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

v2:
- use RSC != 0, not TSC=0 from received frame, as the condition for the
  exception to make this more robust check in case there is a TKIP
  implementation that starts with TSC=0 instead of TSC=1 (the standard
  indicates that TSC is initialized to 1 for new keys, but better cover
  the unexpected case as well)
- clarify the commit message a bit

diff --git a/net/mac80211/tkip.c b/net/mac80211/tkip.c
index 727dc9f3f3b3..9038081070b4 100644
--- a/net/mac80211/tkip.c
+++ b/net/mac80211/tkip.c
@@ -263,9 +263,21 @@ int ieee80211_tkip_decrypt_data(struct arc4_ctx *ctx,
 	if ((keyid >> 6) != key->conf.keyidx)
 		return TKIP_DECRYPT_INVALID_KEYIDX;
 
-	if (rx_ctx->ctx.state != TKIP_STATE_NOT_INIT &&
-	    (iv32 < rx_ctx->iv32 ||
-	     (iv32 == rx_ctx->iv32 && iv16 <= rx_ctx->iv16)))
+	/* Reject replays if the received TSC is smaller than or equal to the
+	 * last received value in a valid message, but with an exception for
+	 * the case where a new key has been set and no valid frame using that
+	 * key has yet received and the local RSC was initialized to 0. This
+	 * exception allows the very first frame sent by the transmitter to be
+	 * accepted even if that transitter were to use TSC 0 (IEEE 802.11
+	 * described TSC to be initialized to 1 whenever a new key is taken into
+	 * use).
+	 */
+	if (iv32 < rx_ctx->iv32 ||
+	    (iv32 == rx_ctx->iv32 &&
+	     (iv16 < rx_ctx->iv16 ||
+	      (iv16 == rx_ctx->iv16 &&
+	       (rx_ctx->iv32 || rx_ctx->iv16 ||
+		rx_ctx->ctx.state != TKIP_STATE_NOT_INIT)))))
 		return TKIP_DECRYPT_REPLAY;
 
 	if (only_iv) {
-- 
2.20.1