linux-kernel - Re: [PATCH v2 0/3] Refactor __kmem_cache

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Message-ID: <Y2PA6JBklwwtLlST@hyeyoo>
Date:   Thu, 3 Nov 2022 22:23:52 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Liu Shixin <liushixin2@...wei.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] Refactor __kmem_cache_create() and fix memory leak

On Wed, Nov 02, 2022 at 04:53:08PM +0800, Liu Shixin wrote:
> On 2022/11/2 15:46, Hyeonggon Yoo wrote:
> > On Mon, Oct 31, 2022 at 09:47:44PM +0800, Liu Shixin wrote:
> >> I found a memory leak of kobj->name in sysfs_slab_add() which is introduced
> >> by 80da026a8e5d ("mm/slub: fix slab double-free in case of duplicate sysfs filename").
> >> Following the rules stated in the comment for kobject_init_and_add():
> > Thank you for reporting this! Indeed it seems tried to fix double free but
> > introduced a leak.
> >
> >>  If this function returns an error, kobject_put() must be called to
> >>  properly clean up the memory associated with the object.
> >>
> >> We should use kobject_put() to free kobject.
> > But what to do if a cache is created early and later sysfs_slab_add() failed?
> > (Which is unlikely on normal condition)
> >
> > With this series it introduces use-after-free if sysfs_slab_add() in
> > slab_sysfs_init() failed.  Should we just call BUG() or something like that?
>
> Thanks for your discovery, what I missed.

You're welcome.

> I prefer to panic directly, just as create_boot_cache() does.

IMHO that should be nothing serious. but let's hear maintainers' opinion.

> Of couse, if you want the system to continue booting, I think it's possible to distinguish them
> by slab_state.

I'm afraid to make it more complex :(

> Looking forward to your advice.
> Thanks,
> >
> >> But we can't simply add kobject_put() since it will free kmem_cache too.
> >> If we use kobject_put(), we need to skip other release functions.
> >>
> >> In this series, We refactor the code to separate sysfs_slab_add() and
> >> debugfs_slab_add() from __kmem_cache_create(), and then use kobject_put()
> >> to free kobject in sysfs_slab_add(). This can fix the memory leak of
> >> kobject->name.
> >>
> >> v1->v2: Fix build error reported by kernel test robot <lkp@...el.com>.
> >>
> >> Liu Shixin (3):
> >>   mm/slab_common: Move cache_name to create_cache()
> >>   mm/slub: Refactor __kmem_cache_create()
> >>   mm/slub: Fix memory leak of kobj->name in sysfs_slab_add()
> >>
> >>  include/linux/slub_def.h | 11 +++++++++
> >>  mm/slab_common.c         | 44 ++++++++++++++++++----------------
> >>  mm/slub.c                | 52 ++++++++++------------------------------
> >>  3 files changed, 48 insertions(+), 59 deletions(-)
> >>
> >> -- 
> >> 2.25.1
> >>
> 

-- 
Thanks,
Hyeonggon