CVE-2024-49940: l2tp: prevent possible tunnel refcount underflow

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-10-21 · updated 2026-10-30

Description

In the Linux kernel, the following vulnerability has been resolved: l2tp: prevent possible tunnel refcount underflow When a session is created, it sets a backpointer to its tunnel. When the session refcount drops to 0, l2tp_session_free drops the tunnel refcount if session->tunnel is non-NULL. However, session->tunnel is set in l2tp_session_create, before the tunnel refcount is incremented by l2tp_session_register, which leaves a small window where session->tunnel is non-NULL when the tunnel refcount hasn't been bumped. Moving the assignment to l2tp_session_register is trivial but l2tp_session_create calls l2tp_session_set_header_len which uses session->tunnel to get the tunnel's encap. Add an encap arg to l2tp_session_set_header_len to avoid using session->tunnel. If l2tpv3 sessions have colliding IDs, it is possible for l2tp_v3_session_get to race with l2tp_session_register and fetch a session which doesn't yet have session->tunnel set. Add a check for this case.

Affected products

Linux kernel: before 6.11.3 — fixed in 6.11.3
Linux: from 3.2.99 before 3.3; from 3.16.54 before 3.17; from 4.4.225 before 4.5; from 4.9.225 before 4.10; from 4.14.182 before 4.15 — fixed in 3.3, 3.17, 4.5, 4.10, 4.15

How to fix

Upgrade to a fixed version: Linux kernel 6.11.3; Linux 3.3, 3.17, 4.5, 4.10. Check the vendor advisory for mitigations.

References

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