l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname

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.