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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: 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 --- 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;