linux-kernel - Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg

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: <vdngz65wojqalim4xb6vxb6k7upbzmkfoogmbuswc4lowju3ke@32hgwjlzxibr>
Date: Tue, 14 Oct 2025 13:14:47 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Hao Ge <hao.ge@...ux.dev>, Vlastimil Babka <vbabka@...e.cz>, 
	Alexei Starovoitov <ast@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Harry Yoo <harry.yoo@...cle.com>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL
 in folio_memcg_kmem

On Tue, Oct 14, 2025 at 09:12:43AM -0700, Suren Baghdasaryan wrote:
> On Tue, Oct 14, 2025 at 8:28 AM Hao Ge <hao.ge@...ux.dev> wrote:
> >
> > From: Hao Ge <gehao@...inos.cn>
> >
> > Since OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS currently share
> > the same bit position, we cannot determine whether memcg_data still
> > points to the slabobj_ext vector simply by checking
> > folio->memcg_data & MEMCG_DATA_OBJEXTS.
> >
> > If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> > and during the release of the associated folio, the BUG check is triggered
> > because it was mistakenly assumed that a valid folio->memcg_data
> > was not cleared before freeing the folio.
> >
> > So let's check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem.
> >
> > Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL")
> > Suggested-by: Harry Yoo <harry.yoo@...cle.com>
> > Signed-off-by: Hao Ge <gehao@...inos.cn>
> 
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> 
> nit: I think it would be helpful if the changelog explained why we
> need the additional check. We can have the same bit set in two
> different situations:
> 1. object extension vector allocation failure;
> 2. memcg_data pointing to a valid mem_cgroup.
> To distinguish between them, we need to check not only the bit itself
> but also the rest of this field. If the rest is NULL, we have case 1,
> otherwise case 2.

With Suren's suggestion, you can add:

Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>