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Date:   Thu, 14 Jul 2022 11:15:59 +0200 (CEST)
From:   Christoph Lameter <cl@gentwo.de>
To:     Marco Elver <elver@google.com>
cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Joe Perches <joe@perches.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        Matthew WilCox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 16/16] mm/sl[au]b: check if large object is valid in
 __ksize()
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On Wed, 13 Jul 2022, Marco Elver wrote:

> We shouldn't crash, so it should be WARN(), but also returning
> PAGE_SIZE is bad. The intuition behind returning 0 is to try and make
> the buggy code cause less harm to the rest of the kernel.
>
> >From [1]:
>
> > Similarly, if you are able to tell if the passed pointer is not a
> > valid object some other way, you can do something better - namely,
> > return 0. The intuition here is that the caller has a pointer to an
> > invalid object, and wants to use ksize() to determine its size, and
> > most likely access all those bytes. Arguably, at that point the kernel
> > is already in a degrading state. But we can try to not let things get
> > worse by having ksize() return 0, in the hopes that it will stop
> > corrupting more memory. It won't work in all cases, but should avoid
> > things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size
> > bounds the memory accessed corrupting random memory.

"in the hopes that it will stop corrupting memory"!!!???

Do a BUG() then and definitely stop all chances of memory corruption.