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

[PATCH] 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 counter 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. There cannot be previously transmitted
frames in such a case, so there cannot be a replay attack either.

Signed-off-by: Jouni Malinen <j@w1.fi>
---
 net/mac80211/tkip.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/mac80211/tkip.c b/net/mac80211/tkip.c
index 727dc9f3f3b3..7a62b3aaf248 100644
--- a/net/mac80211/tkip.c
+++ b/net/mac80211/tkip.c
@@ -263,9 +263,17 @@ 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, but not yet fully initialized
+	 * and the received value is 0. This exception allows the very first
+	 * frame sent by the transmitter to be accepted.
+	 */
+	if (iv32 < rx_ctx->iv32 ||
+	    (iv32 == rx_ctx->iv32 &&
+	     (iv16 < rx_ctx->iv16 ||
+	      (iv16 == rx_ctx->iv16 &&
+	       (iv32 || iv16 || rx_ctx->ctx.state != TKIP_STATE_NOT_INIT)))))
 		return TKIP_DECRYPT_REPLAY;
 
 	if (only_iv) {
-- 
2.20.1