linux-kernel - [PATCH v2 1/1] Bluetooth: L2CAP: Fix use-after-free in l2cap_unregister

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Message-Id: <20251106182016.26508-1-ssranevjti@gmail.com>
Date: Thu,  6 Nov 2025 23:50:16 +0530
From: ssrane_b23@...vjti.ac.in
To: linux-bluetooth@...r.kernel.org
Cc: luiz.dentz@...il.com,
	marcel@...tmann.org,
	johan.hedberg@...il.com,
	pav@....fi,
	linux-kernel@...r.kernel.org,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	syzbot+14b6d57fb728e27ce23c@...kaller.appspotmail.com,
	Shaurya Rane <ssrane_b23@...vjti.ac.in>
Subject: [PATCH v2 1/1] Bluetooth: L2CAP: Fix use-after-free in l2cap_unregister_user

From: Shaurya Rane <ssrane_b23@...vjti.ac.in>

After commit ab4eedb790ca ("Bluetooth: L2CAP: Fix corrupted list in
hci_chan_del"), l2cap_conn_del() uses conn->lock to protect access to
conn->users and conn->hchan. However, l2cap_register_user() and
l2cap_unregister_user() still use hci_dev_lock(), creating a race
condition where these functions can access conn->users and conn->hchan
concurrently with l2cap_conn_del().

This can lead to use-after-free and list corruption bugs, as reported
by syzbot.

Fix this by changing l2cap_register_user() and l2cap_unregister_user()
to use conn->lock instead of hci_dev_lock(), ensuring consistent locking
for the l2cap_conn structure.

Reported-by: syzbot+14b6d57fb728e27ce23c@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=14b6d57fb728e27ce23c
Fixes: ab4eedb790ca ("Bluetooth: L2CAP: Fix corrupted list in hci_chan_del")

Changes in v2:
 - Replaced hci_dev_lock()/unlock() with mutex_lock()/unlock(&conn->lock)
   in both l2cap_register_user() and l2cap_unregister_user().
 - Updated comments to match current locking rules.
 - Removed unnecessary hci_dev_hold()/hci_dev_put() usage.

Signed-off-by: Shaurya Rane <ssrane_b23@...vjti.ac.in>
---
 net/bluetooth/l2cap_core.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index d08320380ad6..29e78801c507 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1685,17 +1685,15 @@ static void l2cap_info_timeout(struct work_struct *work)
 
 int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
 {
-	struct hci_dev *hdev = conn->hcon->hdev;
 	int ret;
 
 	/* We need to check whether l2cap_conn is registered. If it is not, we
-	 * must not register the l2cap_user. l2cap_conn_del() is unregisters
-	 * l2cap_conn objects, but doesn't provide its own locking. Instead, it
-	 * relies on the parent hci_conn object to be locked. This itself relies
-	 * on the hci_dev object to be locked. So we must lock the hci device
-	 * here, too. */
+	 * must not register the l2cap_user. l2cap_conn_del() unregisters
+	 * l2cap_conn objects under conn->lock, and we use the same lock here
+	 * to protect access to conn->users and conn->hchan.
+	 */
 
-	hci_dev_lock(hdev);
+	mutex_lock(&conn->lock);
 
 	if (!list_empty(&user->list)) {
 		ret = -EINVAL;
@@ -1716,16 +1714,14 @@ int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user)
 	ret = 0;
 
 out_unlock:
-	hci_dev_unlock(hdev);
+	mutex_unlock(&conn->lock);
 	return ret;
 }
 EXPORT_SYMBOL(l2cap_register_user);
 
 void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
 {
-	struct hci_dev *hdev = conn->hcon->hdev;
-
-	hci_dev_lock(hdev);
+	mutex_lock(&conn->lock);
 
 	if (list_empty(&user->list))
 		goto out_unlock;
@@ -1734,7 +1730,7 @@ void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user)
 	user->remove(conn, user);
 
 out_unlock:
-	hci_dev_unlock(hdev);
+	mutex_unlock(&conn->lock);
 }
 EXPORT_SYMBOL(l2cap_unregister_user);
 
-- 
2.34.1