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 <[email protected]>
---
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