In the Linux kernel, the following vulnerability has been resolved:
l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname
A reader in l2tp_session_get_by_ifname() can return a pointer to a
session whose refcount has reached zero. The getter takes its
reference with plain refcount_inc(), but every other session getter
in the same file (l2tp_v2_session_get, l2tp_v3_session_get, and the
corresponding _get_next variants) uses refcount_inc_not_zero()
because the IDR/RCU lookup can race with refcount_dec_and_test() ->
l2tp_session_free() -> kfree_rcu(). The ifname getter is the only
outlier; the inconsistency was raised on-list after 979c017803c4
("l2tp: use list_del_rcu in l2tp_session_unhash").
A reader inside rcu_read_lock_bh() that matches session->ifname can
be preempted between the strcmp() and the refcount_inc(). If the
last reference drops on another CPU in that window, the reader's
refcount_inc() runs on a counter that has reached zero. refcount_t
catches the addition-on-zero, prints "refcount_t: addition on 0;
use-after-free", saturates the counter, and returns the saturated
pointer to the caller. Session memory is held live by the in-flight
RCU read section, but the kfree_rcu() callback queued from
l2tp_session_free() will free it once the grace period closes; a
caller that dereferences the returned session past that point hits
a slab-use-after-free. On PREEMPT_RT local_bh_disable() is a per-CPU
sleeping lock and the preemption window is real; on stock PREEMPT
kernels local_bh_disable() is a preempt_count increment that closes
the cross-CPU race in practice (see below).
Use refcount_inc_not_zero() and continue the list walk on failure,
matching the other session getters in the file. The ifname getter
is the only session getter in net/l2tp/ that still uses the bare
refcount_inc() pattern; this change restores file-internal
consistency. The success path is unchanged.
References
Configurations
Configuration 1 (hide)
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History
30 Oct 2026, 14:29
| Type | Values Removed | Values Added |
|---|---|---|
| References | () https://git.kernel.org/stable/c/05f95729ca844704d15e49ce14868af4b403b32b - Patch | |
| References | () https://git.kernel.org/stable/c/782d60a6596aee9b29c2eecfa70033899278bf65 - Patch | |
| References | () https://git.kernel.org/stable/c/947013fd7c8c35dd5856557b215840098a3f67f8 - Patch | |
| References | () https://git.kernel.org/stable/c/ee80455feffb9cb62b5b58715cabeff495e666b2 - Patch | |
| CWE | CWE-416 | |
| CPE | cpe:2.3:o:linux:linux_kernel:7.1:rc4:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:7.1:rc1:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:7.1:rc2:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:7.1:rc5:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:7.1:rc3:*:*:*:*:*:* |
|
| First Time |
Linux linux Kernel
Linux |
Information
Published : 2026-07-19 16:17
Updated : 2026-10-30 14:29
NVD link : CVE-2026-63918
Mitre link : CVE-2026-63918
CVE.ORG link : CVE-2026-63918
JSON object : View
Products Affected
linux
- linux_kernel
CWE
CWE-416
Use After Free