From: Sasha Levin <sashal@kernel.org>
To: stable@vger.kernel.org
Cc: Devin Wittmayer <lucid_duck@justthetip.ca>,
	Johannes Berg <johannes.berg@intel.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.1.y 3/3] wifi: mac80211: refuse to make a monitor active when it has no queue
Date: Tue, 22 Sep 2026 21:39:55 -0400
Message-ID: <20260923013955.878331-3-sashal@kernel.org>
In-Reply-To: <20260923013955.878331-1-sashal@kernel.org>
References: <2026092223-support-ladylike-66dd@gregkh>
 <20260923013955.878331-1-sashal@kernel.org>
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From: Devin Wittmayer <lucid_duck@justthetip.ca>

[ Upstream commit 2b04d6556964ae9f89819b86a0a7801e39c3aae5 ]

A monitor interface only gets a TXQ if it's created active, and one can't
be added later. Setting the flag on a down interface is still allowed, so
the driver is handed a monitor with no queue. ath9k dereferences it:

  BUG: kernel NULL pointer dereference, address: 0000000000000066
  RIP: 0010:ath_tx_node_init+0x49/0x170 [ath9k]
   ath9k_add_interface+0x10c/0x140 [ath9k]
   drv_add_interface+0x54/0x250 [mac80211]
   ieee80211_do_open+0x32f/0x800 [mac80211]

Reached with CAP_NET_ADMIN by "iw dev X set monitor active" followed by
"ip link set X up". RTNL is held, so netlink operations block behind it.

Refuse the flag when there is no queue to give.

Fixes: 79af1f866193 ("mac80211: avoid allocating TXQs that won't be used")
Cc: stable@vger.kernel.org
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Link: https://patch.msgid.link/20260904200338.10829-1-lucid_duck@justthetip.ca
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mac80211/cfg.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
index aa03cebbcf931..a1f7827bdaa92 100644
--- a/net/mac80211/cfg.c
+++ b/net/mac80211/cfg.c
@@ -116,6 +116,10 @@ static int ieee80211_set_mon_options(struct ieee80211_sub_if_data *sdata,
 			return -EBUSY;
 	}
 
+	/* TXQs are reserved in ieee80211_if_add() and cannot be added later */
+	if ((params->flags & MONITOR_FLAG_ACTIVE) && !sdata->vif.txq)
+		return -EOPNOTSUPP;
+
 	/* validate whether MU-MIMO can be configured */
 	if (!ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF) &&
 	    (params->vht_mumimo_groups || params->vht_mumimo_follow_addr))
-- 
2.53.0