From:   Shigeru Yoshida <syoshida@redhat.com>
To:     jchapman@katalix.com, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com
Cc:     gnault@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Shigeru Yoshida <syoshida@redhat.com>
Subject: [PATCH v2] l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()
Date:   Mon, 13 Feb 2023 01:26:23 +0900
Message-Id: <20230212162623.2301597-1-syoshida@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When a file descriptor of pppol2tp socket is passed as file descriptor
of UDP socket, a recursive deadlock occurs in l2tp_tunnel_register().
This situation is reproduced by the following program:

int main(void)
{
	int sock;
	struct sockaddr_pppol2tp addr;

	sock = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
	if (sock < 0) {
		perror("socket");
		return 1;
	}

	addr.sa_family = AF_PPPOX;
	addr.sa_protocol = PX_PROTO_OL2TP;
	addr.pppol2tp.pid = 0;
	addr.pppol2tp.fd = sock;
	addr.pppol2tp.addr.sin_family = PF_INET;
	addr.pppol2tp.addr.sin_port = htons(0);
	addr.pppol2tp.addr.sin_addr.s_addr = inet_addr("192.168.0.1");
	addr.pppol2tp.s_tunnel = 1;
	addr.pppol2tp.s_session = 0;
	addr.pppol2tp.d_tunnel = 0;
	addr.pppol2tp.d_session = 0;

	if (connect(sock, (const struct sockaddr *)&addr, sizeof(addr)) < 0) {
		perror("connect");
		return 1;
	}

	return 0;
}

This program causes the following lockdep warning:

 ============================================
 WARNING: possible recursive locking detected
 6.2.0-rc5-00205-gc96618275234 #56 Not tainted
 --------------------------------------------
 repro/8607 is trying to acquire lock:
 ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: l2tp_tunnel_register+0x2b7/0x11c0

 but task is already holding lock:
 ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(sk_lock-AF_PPPOX);
   lock(sk_lock-AF_PPPOX);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 1 lock held by repro/8607:
  #0: ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30

 stack backtrace:
 CPU: 0 PID: 8607 Comm: repro Not tainted 6.2.0-rc5-00205-gc96618275234 #56
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014
 Call Trace:
  <TASK>
  dump_stack_lvl+0x100/0x178
  __lock_acquire.cold+0x119/0x3b9
  ? lockdep_hardirqs_on_prepare+0x410/0x410
  lock_acquire+0x1e0/0x610
  ? l2tp_tunnel_register+0x2b7/0x11c0
  ? lock_downgrade+0x710/0x710
  ? __fget_files+0x283/0x3e0
  lock_sock_nested+0x3a/0xf0
  ? l2tp_tunnel_register+0x2b7/0x11c0
  l2tp_tunnel_register+0x2b7/0x11c0
  ? sprintf+0xc4/0x100
  ? l2tp_tunnel_del_work+0x6b0/0x6b0
  ? debug_object_deactivate+0x320/0x320
  ? lockdep_init_map_type+0x16d/0x7a0
  ? lockdep_init_map_type+0x16d/0x7a0
  ? l2tp_tunnel_create+0x2bf/0x4b0
  ? l2tp_tunnel_create+0x3c6/0x4b0
  pppol2tp_connect+0x14e1/0x1a30
  ? pppol2tp_put_sk+0xd0/0xd0
  ? aa_sk_perm+0x2b7/0xa80
  ? aa_af_perm+0x260/0x260
  ? bpf_lsm_socket_connect+0x9/0x10
  ? pppol2tp_put_sk+0xd0/0xd0
  __sys_connect_file+0x14f/0x190
  __sys_connect+0x133/0x160
  ? __sys_connect_file+0x190/0x190
  ? lockdep_hardirqs_on+0x7d/0x100
  ? ktime_get_coarse_real_ts64+0x1b7/0x200
  ? ktime_get_coarse_real_ts64+0x147/0x200
  ? __audit_syscall_entry+0x396/0x500
  __x64_sys_connect+0x72/0xb0
  do_syscall_64+0x38/0xb0
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

This patch fixes the issue by getting/creating the tunnel before
locking the pppol2tp socket.

Fixes: 0b2c59720e65 ("l2tp: close all race conditions in l2tp_tunnel_register()")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
---
v2: Get/create the tunnel before locking the pppol2tp socket suggested
    by Guillaume Nault.
v1: https://lore.kernel.org/all/20230130154438.1373750-1-syoshida@redhat.com/

 net/l2tp/l2tp_ppp.c | 118 ++++++++++++++++++++++++--------------------
 1 file changed, 64 insertions(+), 54 deletions(-)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index db2e584c625e..68d02e217ca3 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -650,6 +650,65 @@ static int pppol2tp_tunnel_mtu(const struct l2tp_tunnel *tunnel)
 	return mtu - PPPOL2TP_HEADER_OVERHEAD;
 }
 
+static struct l2tp_tunnel *pppol2tp_tunnel_get(struct net *net,
+					       struct l2tp_connect_info *info,
+					       bool *new_tunnel)
+{
+	struct l2tp_tunnel *tunnel;
+	int error;
+
+	*new_tunnel = false;
+
+	tunnel = l2tp_tunnel_get(net, info->tunnel_id);
+
+	/* Special case: create tunnel context if session_id and
+	 * peer_session_id is 0. Otherwise look up tunnel using supplied
+	 * tunnel id.
+	 */
+	if (!info->session_id && !info->peer_session_id) {
+		if (!tunnel) {
+			struct l2tp_tunnel_cfg tcfg = {
+				.encap = L2TP_ENCAPTYPE_UDP,
+			};
+
+			/* Prevent l2tp_tunnel_register() from trying to set up
+			 * a kernel socket.
+			 */
+			if (info->fd < 0)
+				return ERR_PTR(-EBADF);
+
+			error = l2tp_tunnel_create(info->fd,
+						   info->version,
+						   info->tunnel_id,
+						   info->peer_tunnel_id, &tcfg,
+						   &tunnel);
+			if (error < 0)
+				return ERR_PTR(error);
+
+			l2tp_tunnel_inc_refcount(tunnel);
+			error = l2tp_tunnel_register(tunnel, net, &tcfg);
+			if (error < 0) {
+				kfree(tunnel);
+				return ERR_PTR(error);
+			}
+
+			*new_tunnel = true;
+		}
+	} else {
+		/* Error if we can't find the tunnel */
+		if (!tunnel)
+			return ERR_PTR(-ENOENT);
+
+		/* Error if socket is not prepped */
+		if (!tunnel->sock) {
+			l2tp_tunnel_dec_refcount(tunnel);
+			return ERR_PTR(-ENOENT);
+		}
+	}
+
+	return tunnel;
+}
+
 /* connect() handler. Attach a PPPoX socket to a tunnel UDP socket
  */
 static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
@@ -663,7 +722,6 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	struct pppol2tp_session *ps;
 	struct l2tp_session_cfg cfg = { 0, };
 	bool drop_refcnt = false;
-	bool drop_tunnel = false;
 	bool new_session = false;
 	bool new_tunnel = false;
 	int error;
@@ -672,6 +730,10 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	if (error < 0)
 		return error;
 
+	tunnel = pppol2tp_tunnel_get(sock_net(sk), &info, &new_tunnel);
+	if (IS_ERR(tunnel))
+		return PTR_ERR(tunnel);
+
 	lock_sock(sk);
 
 	/* Check for already bound sockets */
@@ -689,57 +751,6 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	if (!info.tunnel_id)
 		goto end;
 
-	tunnel = l2tp_tunnel_get(sock_net(sk), info.tunnel_id);
-	if (tunnel)
-		drop_tunnel = true;
-
-	/* Special case: create tunnel context if session_id and
-	 * peer_session_id is 0. Otherwise look up tunnel using supplied
-	 * tunnel id.
-	 */
-	if (!info.session_id && !info.peer_session_id) {
-		if (!tunnel) {
-			struct l2tp_tunnel_cfg tcfg = {
-				.encap = L2TP_ENCAPTYPE_UDP,
-			};
-
-			/* Prevent l2tp_tunnel_register() from trying to set up
-			 * a kernel socket.
-			 */
-			if (info.fd < 0) {
-				error = -EBADF;
-				goto end;
-			}
-
-			error = l2tp_tunnel_create(info.fd,
-						   info.version,
-						   info.tunnel_id,
-						   info.peer_tunnel_id, &tcfg,
-						   &tunnel);
-			if (error < 0)
-				goto end;
-
-			l2tp_tunnel_inc_refcount(tunnel);
-			error = l2tp_tunnel_register(tunnel, sock_net(sk),
-						     &tcfg);
-			if (error < 0) {
-				kfree(tunnel);
-				goto end;
-			}
-			drop_tunnel = true;
-			new_tunnel = true;
-		}
-	} else {
-		/* Error if we can't find the tunnel */
-		error = -ENOENT;
-		if (!tunnel)
-			goto end;
-
-		/* Error if socket is not prepped */
-		if (!tunnel->sock)
-			goto end;
-	}
-
 	if (tunnel->peer_tunnel_id == 0)
 		tunnel->peer_tunnel_id = info.peer_tunnel_id;
 
@@ -840,8 +851,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	}
 	if (drop_refcnt)
 		l2tp_session_dec_refcount(session);
-	if (drop_tunnel)
-		l2tp_tunnel_dec_refcount(tunnel);
+	l2tp_tunnel_dec_refcount(tunnel);
 	release_sock(sk);
 
 	return error;
-- 
2.39.0