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CVE Record

CVE-2023-53809: l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2023-53809 is a Linux kernel L2TP issue where a specially arranged PPPoL2TP socket can trigger recursive locking and deadlock. The business impact is likely availability-related on systems using affected Linux kernels with L2TP/PPPoL2TP enabled. The provided sources do not show active exploitation or a CVSS score.

Executive priority

Treat as a moderate availability risk unless L2TP is broadly used on critical Linux systems. Patch through normal kernel maintenance, with faster action for VPN, networking, or edge systems where L2TP support is enabled.

Technical view

The flaw is in l2tp_tunnel_register(). Passing a PPPoL2TP socket descriptor where UDP socket handling is expected can cause the same AF_PPPOX socket lock to be acquired recursively. The kernel fix changes ordering so the tunnel is obtained or created before locking the PPPoL2TP socket.

Likely exposure

Exposure is limited to Linux systems running affected kernel builds with L2TP/PPPoL2TP functionality available. The source bundle names Linux only and includes stable kernel fix references. It does not establish remote reachability, required privileges, or distribution-specific package status.

Exploitation context

CISA KEV is false in the supplied bundle, and no cited source reports exploitation in the wild. The source includes a local reproducer for a deadlock warning, but evidence is insufficient to claim practical weaponization or unauthenticated remote exploitation.

Researcher notes

The core evidence is a kernel lock ordering bug and stable fix. The supplied record lacks CVSS, CWE, exploit-in-the-wild evidence, privilege requirements, and distro package mappings. Avoid overstating impact beyond potential deadlock or availability disruption.

Mitigation direction

  • Update affected Linux kernels using vendor or distribution security guidance.
  • Prioritize systems that enable or expose L2TP/PPPoL2TP functionality.
  • Review kernel stable commits referenced by the CVE for fixed versions.
  • If patching is delayed, check vendor guidance for safe L2TP risk reduction.

Validation and detection

  • Inventory Linux kernel versions across servers, appliances, and containers hosts.
  • Check whether L2TP or PPPoL2TP kernel functionality is present or loaded.
  • Map installed kernels to distribution advisories and referenced stable fixes.
  • Confirm patched systems boot into the corrected kernel, not only install it.
Prepared
Confidence
medium
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2023-53809 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
0ADP providers
6Source links

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux2d77e5c0ad79004b5ef901895437e9cce6dfcc7e, 77e8ed776cdb1a24b2aab8fe7c6f1f154235e1ce, cef0845b6dcfa2f6c2c832e7f9622551456c741d, 0b2c59720e65885a394a017d0cf9cab118914682, 0b2c59720e65885a394a017d0cf9cab118914682, 5.10.166, 5.15.91, 6.1.9unaffected
LinuxLinux6.2, 0, 5.10.173, 5.15.99, 6.1.16, 6.2.3, 6.3affected
Weakness

CWE details

No CWE listed

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