{"id":"CVE-2023-53809","summary":"l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()\n\nWhen a file descriptor of pppol2tp socket is passed as file descriptor\nof UDP socket, a recursive deadlock occurs in l2tp_tunnel_register().\nThis situation is reproduced by the following program:\n\nint main(void)\n{\n\tint sock;\n\tstruct sockaddr_pppol2tp addr;\n\n\tsock = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);\n\tif (sock \u003c 0) {\n\t\tperror(\"socket\");\n\t\treturn 1;\n\t}\n\n\taddr.sa_family = AF_PPPOX;\n\taddr.sa_protocol = PX_PROTO_OL2TP;\n\taddr.pppol2tp.pid = 0;\n\taddr.pppol2tp.fd = sock;\n\taddr.pppol2tp.addr.sin_family = PF_INET;\n\taddr.pppol2tp.addr.sin_port = htons(0);\n\taddr.pppol2tp.addr.sin_addr.s_addr = inet_addr(\"192.168.0.1\");\n\taddr.pppol2tp.s_tunnel = 1;\n\taddr.pppol2tp.s_session = 0;\n\taddr.pppol2tp.d_tunnel = 0;\n\taddr.pppol2tp.d_session = 0;\n\n\tif (connect(sock, (const struct sockaddr *)&addr, sizeof(addr)) \u003c 0) {\n\t\tperror(\"connect\");\n\t\treturn 1;\n\t}\n\n\treturn 0;\n}\n\nThis program causes the following lockdep warning:\n\n ============================================\n WARNING: possible recursive locking detected\n 6.2.0-rc5-00205-gc96618275234 #56 Not tainted\n --------------------------------------------\n repro/8607 is trying to acquire lock:\n ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: l2tp_tunnel_register+0x2b7/0x11c0\n\n but task is already holding lock:\n ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30\n\n other info that might help us debug this:\n  Possible unsafe locking scenario:\n\n        CPU0\n        ----\n   lock(sk_lock-AF_PPPOX);\n   lock(sk_lock-AF_PPPOX);\n\n  *** DEADLOCK ***\n\n  May be due to missing lock nesting notation\n\n 1 lock held by repro/8607:\n  #0: ffff8880213c8130 (sk_lock-AF_PPPOX){+.+.}-{0:0}, at: pppol2tp_connect+0xa82/0x1a30\n\n stack backtrace:\n CPU: 0 PID: 8607 Comm: repro Not tainted 6.2.0-rc5-00205-gc96618275234 #56\n Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014\n Call Trace:\n  \u003cTASK\u003e\n  dump_stack_lvl+0x100/0x178\n  __lock_acquire.cold+0x119/0x3b9\n  ? lockdep_hardirqs_on_prepare+0x410/0x410\n  lock_acquire+0x1e0/0x610\n  ? l2tp_tunnel_register+0x2b7/0x11c0\n  ? lock_downgrade+0x710/0x710\n  ? __fget_files+0x283/0x3e0\n  lock_sock_nested+0x3a/0xf0\n  ? l2tp_tunnel_register+0x2b7/0x11c0\n  l2tp_tunnel_register+0x2b7/0x11c0\n  ? sprintf+0xc4/0x100\n  ? l2tp_tunnel_del_work+0x6b0/0x6b0\n  ? debug_object_deactivate+0x320/0x320\n  ? lockdep_init_map_type+0x16d/0x7a0\n  ? lockdep_init_map_type+0x16d/0x7a0\n  ? l2tp_tunnel_create+0x2bf/0x4b0\n  ? l2tp_tunnel_create+0x3c6/0x4b0\n  pppol2tp_connect+0x14e1/0x1a30\n  ? pppol2tp_put_sk+0xd0/0xd0\n  ? aa_sk_perm+0x2b7/0xa80\n  ? aa_af_perm+0x260/0x260\n  ? bpf_lsm_socket_connect+0x9/0x10\n  ? pppol2tp_put_sk+0xd0/0xd0\n  __sys_connect_file+0x14f/0x190\n  __sys_connect+0x133/0x160\n  ? __sys_connect_file+0x190/0x190\n  ? lockdep_hardirqs_on+0x7d/0x100\n  ? ktime_get_coarse_real_ts64+0x1b7/0x200\n  ? ktime_get_coarse_real_ts64+0x147/0x200\n  ? __audit_syscall_entry+0x396/0x500\n  __x64_sys_connect+0x72/0xb0\n  do_syscall_64+0x38/0xb0\n  entry_SYSCALL_64_after_hwframe+0x63/0xcd\n\nThis patch fixes the issue by getting/creating the tunnel before\nlocking the pppol2tp socket.","modified":"2026-04-02T09:45:17.401404Z","published":"2025-12-09T00:01:07.120Z","related":["SUSE-SU-2026:0316-1"],"database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2023/53xxx/CVE-2023-53809.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/4a413d360959962995e16a899cf2b9ef53e9fcb9"},{"type":"WEB","url":"https://git.kernel.org/stable/c/4bb736b40475528ac1aa8c98b368563618488a70"},{"type":"WEB","url":"https://git.kernel.org/stable/c/5370647dd745bb3d8f37057006be207ddd8e9314"},{"type":"WEB","url":"https://git.kernel.org/stable/c/9ca5e7ecab064f1f47da07f7c1ddf40e4bc0e5ac"},{"type":"WEB","url":"https://git.kernel.org/stable/c/f6df58aa15f7d469f69b1dd21b001ff483255244"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2023/53xxx/CVE-2023-53809.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2023-53809"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"2d77e5c0ad79004b5ef901895437e9cce6dfcc7e"},{"fixed":"4a413d360959962995e16a899cf2b9ef53e9fcb9"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"77e8ed776cdb1a24b2aab8fe7c6f1f154235e1ce"},{"fixed":"f6df58aa15f7d469f69b1dd21b001ff483255244"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"cef0845b6dcfa2f6c2c832e7f9622551456c741d"},{"fixed":"4bb736b40475528ac1aa8c98b368563618488a70"}]},{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"0b2c59720e65885a394a017d0cf9cab118914682"},{"fixed":"5370647dd745bb3d8f37057006be207ddd8e9314"},{"fixed":"9ca5e7ecab064f1f47da07f7c1ddf40e4bc0e5ac"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2023-53809.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.10.173"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.99"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.16"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.2.3"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2023-53809.json"}}],"schema_version":"1.7.5"}