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  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]
Date:	Sun, 14 Apr 2013 04:35:39 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Anatol Pomozov <anatol.pomozov@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Salman Qazi <sqazi@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] module: Fix race condition between load and unload module

On Fri, Apr 12, 2013 at 04:47:50PM -0700, Linus Torvalds wrote:
> This is a much more generic bug in kobjects, and I would hate to add
> some random workaround for just one case of this bug like you do. The
> more fundamental bug needs to be fixed too.
> 
> I think the more fundamental bugfix is to just fix kobject_get() to
> return NULL if the refcount was zero, because in that case the kobject
> no longer really exists.
> 
> So instead of having
> 
>     kref_get(&kobj->kref);
> 
> it should do
> 
>     if (!atomic_inc_not_zero(&kobj->kref.refcount))
>         kobj = NULL;
> 
> and I think that should fix your race automatically, no? Proper patch
> attached (but TOTALLY UNTESTED - it seems to compile, though).
> 
> The problem is that we lose the warning for when the refcount is zero
> and somebody does a kobject_get(), but that is ok *assuming* that
> people actually check the return value of kobject_get() rather than
> just "know" that if they passed in a non-NULL kobj, they'll get it
> right back.
> 
> Greg - please take a look... I'm adding Al to the discussion too,
> because Al just *loooves* these kinds of races ;)

Unless I'm misreading what's going on, we have the following to thank for that:
        /* remove from sysfs if the caller did not do it */
        if (kobj->state_in_sysfs) {
                pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
                         kobject_name(kobj), kobj);
                kobject_del(kobj);
        }
in kobject_cleanup().  Why don't we require kobject_del() before the final
kobject_put(), if the sucker had been added?  FWIW, I thought it *was*
required all along...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists