mac80211: Fix TKIP replay protection immediately after key setup

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>
Link: https://lore.kernel.org/r/20200107153545.10934-1-j@w1.fi
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --git a/net/mac80211/tkip.c b/net/mac80211/tkip.c
index 727dc9f3f3b3a769d2764c529d407073e1d5d825..e7f57bb18f6e00255e75a465cd7ddb1c7282e6f4 100644 (file)
--- 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 transmitter 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) {