From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 144D32C1595; Sun, 7 Jun 2026 10:40:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780828815; cv=none; b=sbNQwlxq99RqAaDahvnHjECXp+0JsCkMrB3ODZG1QOx2TBanc3BNDWs7o3mF+BTZ6DbV6/mdJ1Anpcve6av8NQo1vfS7QP4ViTNCrOuVrIiw8Zhn1DG4vi75wh/Cqdkz+BRohsEVaPiQc4NWW9Rr/tYJwdPzzFIif3BRu3CkNMU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780828815; c=relaxed/simple; bh=LR6Xk/b89tTZZVTPJBX7vlUHWtEpi391mZP/MUVDN6E=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=XeUwhMkS+78mxP1bDW6Z7hinM2vTfvlKtMa0Wo85s5W33/uhw/z34YEQimkXH0ydbS/HRNYqkdbZLiuPZo8rCgEHhTUlAjctXRNK4UJZIu+7OiWDwilFZbToLmDl3SKR3pRz3ZB4u4LVisv1mErwvM/zuiD86fURlLK87rQfcWQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=uuje+aG0; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="uuje+aG0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5CD561F00893; Sun, 7 Jun 2026 10:40:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1780828814; bh=DFcBprbiiw86PxxBkuMAQjAahDIPF2jsgFVWYjYmqDQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=uuje+aG0g55nFxTZ0/1GIxfgMNJxttAAwhXzTpLibLUOuqdI9A+VcF/sEh9vj+fQU C1y41H+BF3o8fB5m6Jst1qoAnak8R4Y4aEOOKeWtt55bmAay7oY9UR9ZIw08gV87lO p82+jfyL4JEgl0M/VL8Gs2ibHIjbvBpEzCgXL90c= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Michael Bommarito , James Chapman , Simon Horman , Jakub Kicinski Subject: [PATCH 6.18 193/315] l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname Date: Sun, 7 Jun 2026 11:59:40 +0200 Message-ID: <20260607095734.660455347@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260607095727.528828913@linuxfoundation.org> References: <20260607095727.528828913@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Michael Bommarito commit 05f95729ca844704d15e49ce14868af4b403b32b upstream. 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 Signed-off-by: Michael Bommarito Reviewed-by: James Chapman Reviewed-by: Simon Horman Link: https://patch.msgid.link/20260523023423.2568972-1-michael.bommarito@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman --- net/l2tp/l2tp_core.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -441,12 +441,13 @@ struct l2tp_session *l2tp_session_get_by idr_for_each_entry_ul(&pn->l2tp_tunnel_idr, tunnel, tmp, tunnel_id) { if (tunnel) { list_for_each_entry_rcu(session, &tunnel->session_list, list) { - if (!strcmp(session->ifname, ifname)) { - refcount_inc(&session->ref_count); - rcu_read_unlock_bh(); + if (strcmp(session->ifname, ifname)) + continue; + if (!refcount_inc_not_zero(&session->ref_count)) + continue; + rcu_read_unlock_bh(); - return session; - } + return session; } } }