From nobody Thu Apr 2 13:35:44 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D54AC04A95 for ; Sun, 25 Sep 2022 12:29:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230128AbiIYM3p (ORCPT ); Sun, 25 Sep 2022 08:29:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47504 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229505AbiIYM3l (ORCPT ); Sun, 25 Sep 2022 08:29:41 -0400 Received: from www262.sakura.ne.jp (www262.sakura.ne.jp [202.181.97.72]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BCE0D5F42 for ; Sun, 25 Sep 2022 05:29:35 -0700 (PDT) Received: from fsav312.sakura.ne.jp (fsav312.sakura.ne.jp [153.120.85.143]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id 28PCTXIL012665; Sun, 25 Sep 2022 21:29:33 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav312.sakura.ne.jp (F-Secure/fsigk_smtp/550/fsav312.sakura.ne.jp); Sun, 25 Sep 2022 21:29:33 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/550/fsav312.sakura.ne.jp) Received: from [192.168.1.9] (M106072142033.v4.enabler.ne.jp [106.72.142.33]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id 28PCTWSJ012662 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NO); Sun, 25 Sep 2022 21:29:33 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Message-ID: <7f489b14-2fdc-3d91-c87e-6a802bd8592d@I-love.SAKURA.ne.jp> Date: Sun, 25 Sep 2022 21:29:32 +0900 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.3.0 Subject: [PATCH] kernfs: fix UAF race condition in __kernfs_remove() Content-Language: en-US To: Greg Kroah-Hartman , Tejun Heo References: <000000000000646c9605e714ec6e@google.com> Cc: syzbot , syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org, Hillf Danton From: Tetsuo Handa In-Reply-To: <000000000000646c9605e714ec6e@google.com> Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" syzbot is reporting use-after-free read at __kernfs_remove() [1], for commit 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible premature return") missed that we need to keep a ref on "kn" as well as "pos". This race condition happens when two concurrent removers "T1" and "T2" interfere due to kernfs_drain() temporarily dropping kernfs_rwsem. T1: T2: down_write(&root->kernfs_rwsem); do { pos =3D kernfs_leftmost_descendant(kn); kernfs_get(pos); kernfs_drain(pos) { up_write(&root->kernfs_rwsem); down_write(&root->kernfs_rwsem); do { // Removes all children and "kn", but won't // free T1's "pos" and "kn", for T1 has a ref // on T1's "pos", and T1's "pos" in turn keeps // a ref on "kn". pos =3D kernfs_leftmost_descendant(kn); kernfs_put(pos); } while (pos !=3D kn) // Will break. up_write(&root->kernfs_rwsem); down_write(&root->kernfs_rwsem); } // Frees "pos" because this was the last ref, and also frees "kn" // because a ref by "pos" was gone (i.e. "kn" no longer has ref) // via "goto repeat;" inside kernfs_put(). kernfs_put(pos); } while (pos !=3D kn) // Will continue, despite "kn" already freed. Link: https://syzkaller.appspot.com/bug?extid=3D8bee3285b9e190f1509e [1] Reported-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com Fixes: 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible = premature return") Tested-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com Co-developed-by: Hillf Danton Signed-off-by: Hillf Danton Signed-off-by: Tetsuo Handa --- fs/kernfs/dir.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 1cc88ba6de90..effb461d34fa 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -1365,6 +1365,11 @@ static void __kernfs_remove(struct kernfs_node *kn) atomic_add(KN_DEACTIVATED_BIAS, &pos->active); =20 /* deactivate and unlink the subtree node-by-node */ + /* + * kernfs_put(pos) will invoke kernfs_put(kn) if @pos was the last + * reference to @kn. Make sure @kn doesn't go away underneath us. + */ + kernfs_get(kn); do { pos =3D kernfs_leftmost_descendant(kn); =20 @@ -1406,6 +1411,7 @@ static void __kernfs_remove(struct kernfs_node *kn) =20 kernfs_put(pos); } while (pos !=3D kn); + kernfs_put(kn); } =20 /** --=20 2.34.1