linux-kernel - Re: SLUB: sysfs lets root force slab order below required minimum, causing memory corruption

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Date:   Sun, 8 Mar 2020 19:34:24 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     David Rientjes <rientjes@...gle.com>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Vijayanand Jitta <vjitta@...eaurora.org>
Subject: Re: SLUB: sysfs lets root force slab order below required minimum,
 causing memory corruption

On Wed, 4 Mar 2020, David Rientjes wrote:

> I'm not sure how dependent the CONFIG_SLUB_DEBUG users are on being able
> to modify these are runtime (they've been around for 12+ years) but I
> agree that it seems particularly dangerous.

The order of each individual slab page is stored in struct page. That is
why every slub slab page can have a different order. This enabled fallback
to order 0 allocations and also allows a dynamic configuration of the
order at runtime.

> The slub_debug kernel command line options are already pretty
> comprehensive as described by Documentation/vm/slub.rst.  I *think* these
> tunables were primarily introduced for kernel debugging and not general
> purpose, perhaps with the exception of "order".

What do you mean by "general purpose? Certainly the allocator should not
blow up when forcing zero order allocations.

> So I think we may be able to fix "order" with a combination of my patch as
> well as a fix to the freelist randomization and that the others should
> likely be made read only.

Hmmm. races increases as more metadata is added that is depending on the
size of the slab page and the number of objects in it.