Re: [PATCH net] l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname

[Simon Horman <horms@kernel.org>]

On Fri, May 22, 2026 at 10:34:23PM -0400, Michael Bommarito wrote:
> 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.
> 
> Fixes: abe7a1a7d0b6 ("l2tp: improve tunnel/session refcount helpers")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-7
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>

Reviewed-by: Simon Horman <horms@kernel.org>