[PATCH] wifi: mac80211_hwsim: avoid treating MCS as legacy rate index

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

Injected HT and VHT rates store an MCS value in rates[0].idx rather
than an index into the legacy bitrate table. hwsim nevertheless passes
these rates to ieee80211_get_tx_rate() while generating monitor frames
and timestamps.

A crafted injected frame can therefore read beyond the bitrate table.
If the resulting bitrate is zero, mac80211_hwsim_write_tsf() also
divides by zero, as observed by syzbot.

Use ieee80211_get_tx_rate() only for legacy rates. The existing fallback
continues to supply a conservative bitrate where hwsim does not yet
calculate MCS rates.

Fixes: e75129031f1c ("wifi: mac80211_hwsim: move timestamp writing later in the datapath")
Reported-by: syzbot+21629c14aa749636db9d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=21629c14aa749636db9d
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 .../net/wireless/virtual/mac80211_hwsim_main.c    | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/virtual/mac80211_hwsim_main.c b/drivers/net/wireless/virtual/mac80211_hwsim_main.c
index 0dd8a6c85953..4a66272526f3 100644
--- a/drivers/net/wireless/virtual/mac80211_hwsim_main.c
+++ b/drivers/net/wireless/virtual/mac80211_hwsim_main.c
@@ -1324,6 +1324,17 @@ static void mac80211_hwsim_set_tsf(struct ieee80211_hw *hw,
 	}
 }
 
+static struct ieee80211_rate *
+mac80211_hwsim_get_tx_rate(struct ieee80211_hw *hw,
+			   struct ieee80211_tx_info *info)
+{
+	if (info->control.rates[0].flags &
+	    (IEEE80211_TX_RC_MCS | IEEE80211_TX_RC_VHT_MCS))
+		return NULL;
+
+	return ieee80211_get_tx_rate(hw, info);
+}
+
 static void mac80211_hwsim_monitor_rx(struct ieee80211_hw *hw,
 				      struct sk_buff *tx_skb,
 				      struct ieee80211_channel *chan)
@@ -1333,7 +1344,7 @@ static void mac80211_hwsim_monitor_rx(struct ieee80211_hw *hw,
 	struct hwsim_radiotap_hdr *hdr;
 	u16 flags, bitrate;
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx_skb);
-	struct ieee80211_rate *txrate = ieee80211_get_tx_rate(hw, info);
+	struct ieee80211_rate *txrate = mac80211_hwsim_get_tx_rate(hw, info);
 
 	if (!txrate)
 		bitrate = 0;
@@ -1603,7 +1614,7 @@ static void mac80211_hwsim_write_tsf(struct mac80211_hwsim_data *data,
 
 	spin_lock_bh(&data->tsf_offset_lock);
 
-	txrate = ieee80211_get_tx_rate(data->hw, info);
+	txrate = mac80211_hwsim_get_tx_rate(data->hw, info);
 	if (txrate)
 		bitrate = txrate->bitrate;
 
-- 
2.54.0