From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by kanga.kvack.org (Postfix) with ESMTP id 313CB8E0001 for ; Tue, 22 Jan 2019 15:28:26 -0500 (EST) Received: by mail-wr1-f69.google.com with SMTP id l1so12982914wrn.3 for ; Tue, 22 Jan 2019 12:28:26 -0800 (PST) Received: from Galois.linutronix.de (Galois.linutronix.de. [2a01:7a0:2:106d:700::1]) by mx.google.com with ESMTPS id g18si47644729wrx.15.2019.01.22.12.28.24 for (version=TLS1_2 cipher=AES128-SHA bits=128/128); Tue, 22 Jan 2019 12:28:24 -0800 (PST) Date: Tue, 22 Jan 2019 21:28:19 +0100 From: Sebastian Andrzej Siewior Subject: Re: [PATCH] backing-dev: no need to check return value of debugfs_create functions Message-ID: <20190122202817.vypepopx4sd757c3@linutronix.de> References: <20190122152151.16139-8-gregkh@linuxfoundation.org> <20190122160759.mx3h7gjc23zmrvxc@linutronix.de> <20190122162503.GB22548@kroah.com> <20190122171908.c7geuvluezkjp3s7@linutronix.de> <20190122183348.GA31271@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190122183348.GA31271@kroah.com> Sender: owner-linux-mm@kvack.org List-ID: To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, Andrew Morton , Anders Roxell , Arnd Bergmann , Michal Hocko , linux-mm@kvack.org On 2019-01-22 19:33:48 [+0100], Greg Kroah-Hartman wrote: > On Tue, Jan 22, 2019 at 06:19:08PM +0100, Sebastian Andrzej Siewior wrote: > > but if you cat the stats file then it will dereference the bdi struct > > which has been free(), right? > > Maybe, I don't know, your code is long gone, it doesn't matter :) may point is that you may remain with a stats file in debugfs' root folder which you can cat and then crash. > > > But step back, how could that original call be NULL? That only happens > > > if you pass it a bad parent dentry (which you didn't), or the system is > > > totally out of memory (in which case you don't care as everything else > > > is on fire). > > > > debugfs_get_inode() could do -ENOMEM and then the directory creation > > fails with NULL. > > And if that happens, your system has worse problems :) So we care to properly handle -ENOMEM in driver's probe function. Those change find their way to stable kernels. This unhandled -ENOMEM in debugfs_get_inode() will let debugfs_create_dir() reuturn NULL. Then debugfs_create_file() will create the stats in debugfs' root folder. This is a changed behaviour which is not expected. And then on rmmod the stats file is still present and will participate in use-after-free if it is read. > As it's been that way for over a decade, I think we will be fine :) > If it changes in the future, in some way that actually matters, I'll go > back and fix up all of the callers. That is okay then :). I don't mind if the stats file does not show up due to an error on probe. It is debugfs after all. However I don't think that it is okay that the stats file remains in the root folder even after the module has been removed (and access memory that does not belong to it). > thanks, > > greg k-h Sebastian