Date: Fri, 31 Mar 2017 14:33:17 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: Add additional consistency check
Message-Id: <20170331143317.3865149a6b6112f0d1a63499@linux-foundation.org>
In-Reply-To: <20170331164028.GA118828@beast>
References: <20170331164028.GA118828@beast>
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On Fri, 31 Mar 2017 09:40:28 -0700 Kees Cook <keescook@chromium.org> wrote:

> As found in PaX, this adds a cheap check on heap consistency, just to
> notice if things have gotten corrupted in the page lookup.

"As found in PaX" isn't a very illuminating justification for such a
change.  Was there a real kernel bug which this would have exposed, or
what?

> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -384,6 +384,7 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
>  		return s;
>  
>  	page = virt_to_head_page(x);
> +	BUG_ON(!PageSlab(page));
>  	cachep = page->slab_cache;
>  	if (slab_equal_or_root(cachep, s))
>  		return cachep;

BUG_ON might be too severe.  I expect the kindest VM_WARN_ON_ONCE()
would suffice here, but without more details it is hard to say.