linux-kernel - Re: [PATCH 1/2] mm: kfence: fix missing objcg housekeeping for SLAB

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Date:   Mon, 28 Mar 2022 09:36:52 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+f8c45ccc7d5d45fc5965@...kaller.appspotmail.com>
Subject: Re: [PATCH 1/2] mm: kfence: fix missing objcg housekeeping for SLAB

On Mon, Mar 28, 2022 at 5:08 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sat, Mar 26, 2022 at 10:19 PM Muchun Song <songmuchun@...edance.com> wrote:
> >
> > The objcg is not cleared and put for kfence object when it is freed, which
> > could lead to memory leak for struct obj_cgroup and wrong statistics of
> > NR_SLAB_RECLAIMABLE_B or NR_SLAB_UNRECLAIMABLE_B.  Since the last freed
> > object's objcg is not cleared, mem_cgroup_from_obj() could return the wrong
> > memcg when this kfence object, which is not charged to any objcgs, is
> > reallocated to other users.  A real word issue [1] is caused by this bug.
>
> Good that this looks sorted out.
>
> Patch 2/2 seems to still be up in the air. The patch not only causes
> build errors, but it looks really very odd to me.
>
> In particular, you do that loop with
>
>                 __SetPageSlab(&pages[i]);
>
> in kfence_init_pool(), but that is *not* where you set the
> MEMCG_DATA_OBJCGS, and instead do that virt_to_slab(addr) dance later.
>
> That looks very odd to me. I think the two should go hand-in-hand,
> since that __SetPageSlab() really is what makes it a slab thing, and I
> think it should go together with setting the slab state correctly.

Right. It is a little odd. I'll improve it in the next version.

>
> Finally, is there a syzbot report for that second problem?

No. The second bug does not trigger any oops, so it is hard to be seen.
It is just my code review.

>
> Anyway, should I apply this PATCH 1/2 now directly as the solution for
> the dentry issue, or should I wait for that second patch? They seem to
> be related only indirectly, in that the problems were both introduced
> by the same commit.
>

I think you could apply PATCH 1/2 now.  PATCH 2/2 is another issue not
related to dentry issue.

Thanks.