From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,PDS_BTC_ID,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 58746C43603 for ; Wed, 4 Dec 2019 12:31:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AB302084B for ; Wed, 4 Dec 2019 12:31:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575462715; bh=BqmufO685yzxrGMkexQbTJa/wGdgpCJGHEB1cLv5CEg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=FgAAQYb+kbnlpwHvbrZc9TcVzaohDfZqAox293icZXX4rwFjmT4kOhAFn9oMIdg/4 CcxdBErlvxVssbutZ6fCLleHoICECo+RnMw3GdTQP84ZlPNzAPpbpjLY6lFMWVXSZB vVYAPc1FMMpNBq32t6CAXFvaEH6bMGITODHSVWRg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727744AbfLDMby (ORCPT ); Wed, 4 Dec 2019 07:31:54 -0500 Received: from mail.kernel.org ([198.145.29.99]:33050 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726604AbfLDMbx (ORCPT ); Wed, 4 Dec 2019 07:31:53 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B738420833; Wed, 4 Dec 2019 12:31:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575462712; bh=BqmufO685yzxrGMkexQbTJa/wGdgpCJGHEB1cLv5CEg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=w653gOaA/Kti/YNygsEQd45vn9R/cMuK4nEMFi9bhVIt2aDRiaj++apNi/2c0+AVx rIqYyyUrwPaR82ufsCa7CjqBxvo/G7+e6Da6dSkK3q5DUsNQupgkc+fImV4R1GpG+h zaucSmROx+KhRWRIpcfxCSQvwxofT/K9Tm2zGer8= Date: Wed, 4 Dec 2019 13:31:48 +0100 From: Greg KH To: Will Deacon Cc: syzbot , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, hdanton@sina.com, akpm@linux-foundation.org Subject: Re: WARNING: refcount bug in cdev_get Message-ID: <20191204123148.GA3626092@kroah.com> References: <000000000000bf410005909463ff@google.com> <20191204115055.GA24783@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191204115055.GA24783@willie-the-truck> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 04, 2019 at 11:50:56AM +0000, Will Deacon wrote: > Hi all, > > [+Hillf, +akpm, +Greg] > > On Tue, Aug 20, 2019 at 03:58:06PM -0700, syzbot wrote: > > syzbot found the following crash on: > > > > HEAD commit: 2d63ba3e Merge tag 'pm-5.3-rc5' of git://git.kernel.org/pu.. > > git tree: upstream > > console output: https://syzkaller.appspot.com/x/log.txt?x=165d3302600000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=3ff364e429585cf2 > > dashboard link: https://syzkaller.appspot.com/bug?extid=82defefbbd8527e1c2cb > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16c8ab3c600000 > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16be0c4c600000 > > > > Bisection is inconclusive: the bug happens on the oldest tested release. > > > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=11de3622600000 > > console output: https://syzkaller.appspot.com/x/log.txt?x=15de3622600000 > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > Reported-by: syzbot+82defefbbd8527e1c2cb@syzkaller.appspotmail.com > > > > ------------[ cut here ]------------ > > refcount_t: increment on 0; use-after-free. > > WARNING: CPU: 1 PID: 11828 at lib/refcount.c:156 refcount_inc_checked > > lib/refcount.c:156 [inline] > > WARNING: CPU: 1 PID: 11828 at lib/refcount.c:156 > > refcount_inc_checked+0x61/0x70 lib/refcount.c:154 > > Kernel panic - not syncing: panic_on_warn set ... > > [...] > > > RIP: 0010:refcount_inc_checked lib/refcount.c:156 [inline] > > RIP: 0010:refcount_inc_checked+0x61/0x70 lib/refcount.c:154 > > Code: 1d 8e c6 64 06 31 ff 89 de e8 ab 9c 35 fe 84 db 75 dd e8 62 9b 35 fe > > 48 c7 c7 00 05 c6 87 c6 05 6e c6 64 06 01 e8 67 26 07 fe <0f> 0b eb c1 90 90 > > 90 90 90 90 90 90 90 90 90 55 48 89 e5 41 57 41 > > RSP: 0018:ffff8880907d78b8 EFLAGS: 00010282 > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 > > RDX: 0000000000000000 RSI: ffffffff815c2466 RDI: ffffed10120faf09 > > RBP: ffff8880907d78c8 R08: ffff8880a771a200 R09: fffffbfff134ae48 > > R10: fffffbfff134ae47 R11: ffffffff89a5723f R12: ffff88809ea2bb80 > > R13: 0000000000000000 R14: ffff88809ff6cd40 R15: ffff8880a1c56480 > > kref_get include/linux/kref.h:45 [inline] > > kobject_get+0x66/0xc0 lib/kobject.c:644 > > cdev_get+0x60/0xb0 fs/char_dev.c:355 > > chrdev_open+0xb0/0x6b0 fs/char_dev.c:400 > > do_dentry_open+0x4df/0x1250 fs/open.c:797 > > vfs_open+0xa0/0xd0 fs/open.c:906 > > do_last fs/namei.c:3416 [inline] > > path_openat+0x10e9/0x4630 fs/namei.c:3533 > > do_filp_open+0x1a1/0x280 fs/namei.c:3563 > > do_sys_open+0x3fe/0x5d0 fs/open.c:1089 > > FWIW, we've run into this same crash on arm64 so it would be nice to see it > fixed upstream. It looks like Hillf's reply (which included a patch) didn't > make it to the kernel mailing lists for some reason, but it is available > here: > > https://groups.google.com/forum/#!original/syzkaller-bugs/PnQNxBrWv_8/X1ygj8d8DgAJ No one is going to go and dig a patch out of google groups :( > A simpler fix would just be to use kobject_get_unless_zero() directly in > cdev_get(), but that looks odd in this specific case because chrdev_open() > holds the 'cdev_lock' and you'd hope that finding the kobject in the inode > with that held would mean that it's not being freed at the same time. When using kref_get_unless_zero() that implies that a lock is not being used and you are relying on the kobject only instead. But I thought we had a lock in play here, so why would changing this actually fix anything? This code hasn't changed in 15+ years, what suddenly changed that causes problems here? thanks, greg k-h