Bluetooth: hci_conn: Fix possible UAF

This fixes the following trace:

==================================================================
BUG: KASAN: slab-use-after-free in hci_conn_del+0xba/0x3a0
Write of size 8 at addr ffff88800208e9c8 by task iso-tester/31

CPU: 0 PID: 31 Comm: iso-tester Not tainted 6.3.0-rc2-g991aa4a69a47
 #4716
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.1-2.fc36
04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x1d/0x70
 print_report+0xce/0x610
 ? __virt_addr_valid+0xd4/0x150
 ? hci_conn_del+0xba/0x3a0
 kasan_report+0xdd/0x110
 ? hci_conn_del+0xba/0x3a0
 hci_conn_del+0xba/0x3a0
 hci_conn_hash_flush+0xf2/0x120
 hci_dev_close_sync+0x388/0x920
 hci_unregister_dev+0x122/0x260
 vhci_release+0x4f/0x90
 __fput+0x102/0x430
 task_work_run+0xf1/0x160
 ? __pfx_task_work_run+0x10/0x10
 ? mark_held_locks+0x24/0x90
 exit_to_user_mode_prepare+0x170/0x180
 syscall_exit_to_user_mode+0x19/0x50
 do_syscall_64+0x4e/0x90
 entry_SYSCALL_64_after_hwframe+0x70/0xda

Fixes: 0f00cd322d ("Bluetooth: Free potentially unfreed SCO connection")
Link: https://syzkaller.appspot.com/bug?extid=8bb72f86fc823817bc5d
Cc: <stable@vger.kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/hci_conn.c

@@ -1068,6 +1068,17 @@ struct hci_conn *hci_conn_add(struct hci_dev *hdev, int type, bdaddr_t *dst,
 	return conn;
 }
 
+static bool hci_conn_unlink(struct hci_conn *conn)
+{
+	if (!conn->link)
+		return false;
+
+	conn->link->link = NULL;
+	conn->link = NULL;
+
+	return true;
+}
+
 int hci_conn_del(struct hci_conn *conn)
 {
 	struct hci_dev *hdev = conn->hdev;
@@ -1079,15 +1090,16 @@ int hci_conn_del(struct hci_conn *conn)
 	cancel_delayed_work_sync(&conn->idle_work);
 
 	if (conn->type == ACL_LINK) {
-		struct hci_conn *sco = conn->link;
+		struct hci_conn *link = conn->link;
-		if (sco) {
+
-			sco->link = NULL;
+		if (link) {
+			hci_conn_unlink(conn);
 			/* Due to race, SCO connection might be not established
 			 * yet at this point. Delete it now, otherwise it is
 			 * possible for it to be stuck and can't be deleted.
 			 */
-			if (sco->handle == HCI_CONN_HANDLE_UNSET)
+			if (link->handle == HCI_CONN_HANDLE_UNSET)
-				hci_conn_del(sco);
+				hci_conn_del(link);
 		}
 
 		/* Unacked frames */
@@ -1103,7 +1115,7 @@ int hci_conn_del(struct hci_conn *conn)
 		struct hci_conn *acl = conn->link;
 
 		if (acl) {
-			acl->link = NULL;
+			hci_conn_unlink(conn);
 			hci_conn_drop(acl);
 		}
 
@@ -2434,6 +2446,12 @@ void hci_conn_hash_flush(struct hci_dev *hdev)
 		c->state = BT_CLOSED;
 
 		hci_disconn_cfm(c, HCI_ERROR_LOCAL_HOST_TERM);
+
+		/* Unlink before deleting otherwise it is possible that
+		 * hci_conn_del removes the link which may cause the list to
+		 * contain items already freed.
+		 */
+		hci_conn_unlink(c);
 		hci_conn_del(c);
 	}
 }