linux-kernel - Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YYFQdWvpXOV4foyS@alley>
Date:   Tue, 2 Nov 2021 15:51:33 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, tj@...nel.org,
        gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        minchan@...nel.org, jeyu@...nel.org, shuah@...nel.org,
        bvanassche@....org, dan.j.williams@...el.com, joe@...ches.com,
        tglx@...utronix.de, keescook@...omium.org, rostedt@...dmis.org,
        linux-spdx@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

On Tue 2021-11-02 15:15:19, Petr Mladek wrote:
> On Tue 2021-10-26 23:37:30, Ming Lei wrote:
> > On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote:
> > > Below are more details about the livepatch code. I hope that it will
> > > help you to see if zram has similar problems or not.
> > > 
> > > We have kobject in three structures: klp_func, klp_object, and
> > > klp_patch, see include/linux/livepatch.h.
> > > 
> > > These structures have to be statically defined in the module sources
> > > because they define what is livepatched, see
> > > samples/livepatch/livepatch-sample.c
> > > 
> > > The kobject is used there to show information about the patch, patched
> > > objects, and patched functions, in sysfs. And most importantly,
> > > the sysfs interface can be used to disable the livepatch.
> > > 
> > > The problem with static structures is that the module must stay
> > > in the memory as long as the sysfs interface exists. It can be
> > > solved in module_exit() callback. It could wait until the sysfs
> > > interface is destroyed.
> > > 
> > > kobject API does not support this scenario. The relase() callbacks
> > 
> > kobject_delete() is for supporting this scenario, that is why we don't
> > need to grab module refcnt before calling show()/store() of the
> > kobject's attributes.
> > 
> > kobject_delete() can be called in module_exit(), then any show()/store()
> > will be done after kobject_delete() returns.
> 
> I am a bit confused. I do not see kobject_delete() anywhere in kernel
> sources.
> 
> I see only kobject_del() and kobject_put(). AFAIK, they do _not_
> guarantee that either the sysfs interface was destroyed or
> the release callbacks were called. For example, see
> schedule_delayed_work(&kobj->release, delay) in kobject_release().

Grr, I always get confused by the code. kobject_del() actually waits
until the sysfs interface gets destroyed. This is why there is
the deadlock.

But kobject_put() is _not_ synchronous. And the comment above
kobject_add() repeat 3 times that kobject_put() must be called
on success:

 * Return: If this function returns an error, kobject_put() must be
 *         called to properly clean up the memory associated with the
 *         object.  Under no instance should the kobject that is passed
 *         to this function be directly freed with a call to kfree(),
 *         that can leak memory.
 *
 *         If this function returns success, kobject_put() must also be called
 *         in order to properly clean up the memory associated with the object.
 *
 *         In short, once this function is called, kobject_put() MUST be called
 *         when the use of the object is finished in order to properly free
 *         everything.

and similar text in Documentation/core-api/kobject.rst

  After a kobject has been registered with the kobject core successfully, it
  must be cleaned up when the code is finished with it.  To do that, call
  kobject_put().

If I read the code correctly then kobject_put() calls kref_put()
that might call kobject_delayed_cleanup(). This function does a lot
of things and need to access struct kobject.

> IMHO, kobject API does not support static structures and module
> removal.

If kobject_put() has to be called also for static structures then
module_exit() must explicitly wait until the clean up is finished.

Best Regards,
Petr