CVE-2023-53809 - Vulnerability Details

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.

No CVSS v4.0

Attack Vector Local

Attack Complexity Low

Privileges Required Low

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00026.

Key SSVC decision points have not yet been added.

Vendors	Products
Linux Subscribe	Linux Kernel Subscribe

No data.

Advisories

No advisories yet.

Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References

Link	Providers
https://git.kernel.org/stable/c/4a413d360959962995e16a899cf2b9ef53e9fcb9
https://git.kernel.org/stable/c/4bb736b40475528ac1aa8c98b368563618488a70
https://git.kernel.org/stable/c/5370647dd745bb3d8f37057006be207ddd8e9314
https://git.kernel.org/stable/c/9ca5e7ecab064f1f47da07f7c1ddf40e4bc0e5ac
https://git.kernel.org/stable/c/f6df58aa15f7d469f69b1dd21b001ff483255244
https://lore.kernel.org/linux-cve-announce/2025120944-CVE-2023-53809-ca78@gregkh/T
https://nvd.nist.gov/vuln/detail/CVE-2023-53809
https://www.cve.org/CVERecord?id=CVE-2023-53809

History

Wed, 10 Dec 2025 12:15:00 +0000

Type	Values Removed	Values Added
References		https://lore.kernel.org/linux-cve-announce/2025120944-CVE-2023-53809-ca78@gregkh/T https://nvd.nist.gov/vuln/detail/CVE-2023-53809 https://www.cve.org/CVERecord?id=CVE-2023-53809
Metrics	threat_severity `None`	cvssV3_1 `{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}` threat_severity `Low`

Tue, 09 Dec 2025 00:30:00 +0000

Type	Values Removed	Values Added
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.
Title		l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/4a413d360959962995e16a899cf2b9ef53e9fcb9 https://git.kernel.org/stable/c/4bb736b40475528ac1aa8c98b368563618488a70 https://git.kernel.org/stable/c/5370647dd745bb3d8f37057006be207ddd8e9314 https://git.kernel.org/stable/c/9ca5e7ecab064f1f47da07f7c1ddf40e4bc0e5ac https://git.kernel.org/stable/c/f6df58aa15f7d469f69b1dd21b001ff483255244

Projects

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MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2025-12-09T00:01:07.120Z

Updated: 2025-12-09T00:01:07.120Z

Reserved: 2025-12-08T23:58:35.276Z

Link: CVE-2023-53809

Vulnrichment

No data.

NVD

Status : Awaiting Analysis

Published: 2025-12-09T01:16:52.940

Modified: 2025-12-09T18:37:13.640

Link: CVE-2023-53809

Redhat

Severity : Low

Publid Date: 2025-12-09T00:00:00Z

Links: CVE-2023-53809 - Bugzilla

OpenCVE Enrichment

No data.

Weaknesses

No weakness.

Metrics

Attack Vector Local

Attack Complexity Low

Privileges Required Low

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

Affected Vendors & Products

Projects

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