l2tp: make session IDR and tunnel session list coherent

Modify l2tp_session_register and l2tp_session_unhash so that the
session IDR and tunnel session lists remain coherent. To do so, hold
the session IDR lock and the tunnel's session list lock when making
any changes to either list.

Without this change, a rare race condition could hit the WARN_ON_ONCE
in l2tp_session_unhash if a thread replaced the IDR entry while
another thread was registering the same ID.

 [ 7126.151795][T17511] WARNING: CPU: 3 PID: 17511 at net/l2tp/l2tp_core.c:1282 l2tp_session_delete.part.0+0x87e/0xbc0
 [ 7126.163754][T17511]  ? show_regs+0x93/0xa0
 [ 7126.164157][T17511]  ? __warn+0xe5/0x3c0
 [ 7126.164536][T17511]  ? l2tp_session_delete.part.0+0x87e/0xbc0
 [ 7126.165070][T17511]  ? report_bug+0x2e1/0x500
 [ 7126.165486][T17511]  ? l2tp_session_delete.part.0+0x87e/0xbc0
 [ 7126.166013][T17511]  ? handle_bug+0x99/0x130
 [ 7126.166428][T17511]  ? exc_invalid_op+0x35/0x80
 [ 7126.166890][T17511]  ? asm_exc_invalid_op+0x1a/0x20
 [ 7126.167372][T17511]  ? l2tp_session_delete.part.0+0x87d/0xbc0
 [ 7126.167900][T17511]  ? l2tp_session_delete.part.0+0x87e/0xbc0
 [ 7126.168429][T17511]  ? __local_bh_enable_ip+0xa4/0x120
 [ 7126.168917][T17511]  l2tp_session_delete+0x40/0x50
 [ 7126.169369][T17511]  pppol2tp_release+0x1a1/0x3f0
 [ 7126.169817][T17511]  __sock_release+0xb3/0x270
 [ 7126.170247][T17511]  ? __pfx_sock_close+0x10/0x10
 [ 7126.170697][T17511]  sock_close+0x1c/0x30
 [ 7126.171087][T17511]  __fput+0x40b/0xb90
 [ 7126.171470][T17511]  task_work_run+0x16c/0x260
 [ 7126.171897][T17511]  ? __pfx_task_work_run+0x10/0x10
 [ 7126.172362][T17511]  ? srso_alias_return_thunk+0x5/0xfbef5
 [ 7126.172863][T17511]  ? do_raw_spin_unlock+0x174/0x230
 [ 7126.173348][T17511]  do_exit+0xaae/0x2b40
 [ 7126.173730][T17511]  ? srso_alias_return_thunk+0x5/0xfbef5
 [ 7126.174235][T17511]  ? __pfx_lock_release+0x10/0x10
 [ 7126.174690][T17511]  ? srso_alias_return_thunk+0x5/0xfbef5
 [ 7126.175190][T17511]  ? do_raw_spin_lock+0x12c/0x2b0
 [ 7126.175650][T17511]  ? __pfx_do_exit+0x10/0x10
 [ 7126.176072][T17511]  ? _raw_spin_unlock_irq+0x23/0x50
 [ 7126.176543][T17511]  do_group_exit+0xd3/0x2a0
 [ 7126.176990][T17511]  __x64_sys_exit_group+0x3e/0x50
 [ 7126.177456][T17511]  x64_sys_call+0x1821/0x1830
 [ 7126.177895][T17511]  do_syscall_64+0xcb/0x250
 [ 7126.178317][T17511]  entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: aa5e17e1 ("l2tp: store l2tpv3 sessions in per-net IDR")
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240718134348.289865-1-jchapman@katalix.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/l2tp/l2tp_core.c

@@ -441,14 +441,15 @@ int l2tp_session_register(struct l2tp_session *session,
 	int err;
 
 	spin_lock_bh(&tunnel->list_lock);
+	spin_lock_bh(&pn->l2tp_session_idr_lock);
+
 	if (!tunnel->acpt_newsess) {
 		err = -ENODEV;
-		goto err_tlock;
+		goto out;
 	}
 
 	if (tunnel->version == L2TP_HDR_VER_3) {
 		session_key = session->session_id;
-		spin_lock_bh(&pn->l2tp_session_idr_lock);
 		err = idr_alloc_u32(&pn->l2tp_v3_session_idr, NULL,
 				    &session_key, session_key, GFP_ATOMIC);
 		/* IP encap expects session IDs to be globally unique, while
@@ -462,43 +463,36 @@ int l2tp_session_register(struct l2tp_session *session,
 			err = l2tp_session_collision_add(pn, session,
 							 other_session);
 		}
-		spin_unlock_bh(&pn->l2tp_session_idr_lock);
 	} else {
 		session_key = l2tp_v2_session_key(tunnel->tunnel_id,
 						  session->session_id);
-		spin_lock_bh(&pn->l2tp_session_idr_lock);
 		err = idr_alloc_u32(&pn->l2tp_v2_session_idr, NULL,
 				    &session_key, session_key, GFP_ATOMIC);
-		spin_unlock_bh(&pn->l2tp_session_idr_lock);
 	}
 
 	if (err) {
 		if (err == -ENOSPC)
 			err = -EEXIST;
-		goto err_tlock;
+		goto out;
 	}
 
 	l2tp_tunnel_inc_refcount(tunnel);
-
 	list_add(&session->list, &tunnel->session_list);
-	spin_unlock_bh(&tunnel->list_lock);
 
-	spin_lock_bh(&pn->l2tp_session_idr_lock);
 	if (tunnel->version == L2TP_HDR_VER_3) {
 		if (!other_session)
 			idr_replace(&pn->l2tp_v3_session_idr, session, session_key);
 	} else {
 		idr_replace(&pn->l2tp_v2_session_idr, session, session_key);
 	}
-	spin_unlock_bh(&pn->l2tp_session_idr_lock);
-
-	trace_register_session(session);
 
-	return 0;
-
-err_tlock:
+out:
+	spin_unlock_bh(&pn->l2tp_session_idr_lock);
 	spin_unlock_bh(&tunnel->list_lock);
 
+	if (!err)
+		trace_register_session(session);
+
 	return err;
 }
 EXPORT_SYMBOL_GPL(l2tp_session_register);
@@ -1260,13 +1254,13 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 		struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);
 		struct l2tp_session *removed = session;
 
-		/* Remove from the per-tunnel list */
 		spin_lock_bh(&tunnel->list_lock);
+		spin_lock_bh(&pn->l2tp_session_idr_lock);
+
+		/* Remove from the per-tunnel list */
 		list_del_init(&session->list);
-		spin_unlock_bh(&tunnel->list_lock);
 
 		/* Remove from per-net IDR */
-		spin_lock_bh(&pn->l2tp_session_idr_lock);
 		if (tunnel->version == L2TP_HDR_VER_3) {
 			if (hash_hashed(&session->hlist))
 				l2tp_session_collision_del(pn, session);
@@ -1280,7 +1274,9 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 					     session_key);
 		}
 		WARN_ON_ONCE(removed && removed != session);
+
 		spin_unlock_bh(&pn->l2tp_session_idr_lock);
+		spin_unlock_bh(&tunnel->list_lock);
 
 		synchronize_rcu();
 	}