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 for Android: free password hash cracker in your pocket
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Thu, 6 Feb 2014 22:06:10 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<penberg@...nel.org>, <akpm@...ux-foundation.org>,
	<rientjes@...gle.com>, <mhocko@...e.cz>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<devel@...nvz.org>
Subject: Re: [PATCH RFC] slub: do not drop slab_mutex for sysfs_slab_{add,remove}

On 02/06/2014 08:22 PM, Christoph Lameter wrote:
> On Thu, 6 Feb 2014, Vladimir Davydov wrote:
>
>> When creating/destroying a kmem cache, we do a lot of work holding the
>> slab_mutex, but we drop it for sysfs_slab_{add,remove} for some reason.
>> Since __kmem_cache_create and __kmem_cache_shutdown are extremely rare,
>> I propose to simplify locking by calling sysfs_slab_{add,remove} w/o
>> dropping the slab_mutex.
> The problem is that sysfs does nasty things like spawning a process in
> user space that may lead to something wanting to create slabs too. The
> module may then hang waiting on the lock ...

Hmm... IIUC the only function of concern is kobject_uevent() -
everything else called from sysfs_slab_{add,remove} is a mix of kmalloc,
kfree, mutex_lock/unlock - in short, nothing dangerous. There we do
call_usermodehelper(), but we do it with UMH_WAIT_EXEC, which means
"wait for exec only, but not for the process to complete". An exec
shouldn't issue any slab-related stuff AFAIU. At least, I tried to run
the patched kernel with lockdep enabled and got no warnings at all when
getting uevents about adding/removing caches. That's why I started to
doubt whether we really need this lock...

Please correct me if I'm wrong.

> I would be very thankful, if you can get that actually working reliably
> without deadlock issues.

If there is no choice rather than moving sysfs_slab_{add,remove} out of
the slab_mutex critical section, I'll have to do it that way. But first
I'd like to make sure it cannot be done with less footprint.

Thanks.
--
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