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Date: Mon, 17 Jul 2017 12:11:40 -0700
From: Kees Cook <>
To: Alexander Popov <>
Cc: Christopher Lameter <>, Matthew Wilcox <>, Pekka Enberg <>, 
	David Rientjes <>, Joonsoo Kim <>, 
	Andrew Morton <>, Linux-MM <>, 
	LKML <>, 
	"" <>
Subject: Re: [PATCH 1/1] mm/slub.c: add a naive detection of double free or corruption

On Mon, Jul 17, 2017 at 12:01 PM, Alexander Popov <> wrote:
> Hello Christopher,
> Thanks for your reply.
> On 17.07.2017 21:04, Christopher Lameter wrote:
>> On Mon, 17 Jul 2017, Matthew Wilcox wrote:
>>> On Mon, Jul 17, 2017 at 07:45:07PM +0300, Alexander Popov wrote:
>>>> Add an assertion similar to "fasttop" check in GNU C Library allocator:
>>>> an object added to a singly linked freelist should not point to itself.
>>>> That helps to detect some double free errors (e.g. CVE-2017-2636) without
>>>> slub_debug and KASAN. Testing with hackbench doesn't show any noticeable
>>>> performance penalty.
>>>>  {
>>>> +   BUG_ON(object == fp); /* naive detection of double free or corruption */
>>>>     *(void **)(object + s->offset) = fp;
>>>>  }
>>> Is BUG() the best response to this situation?  If it's a corruption, then
>>> yes, but if we spot a double-free, then surely we should WARN() and return
>>> without doing anything?
>> The double free debug checking already does the same thing in a more
>> thourough way (this one only checks if the last free was the same
>> address). So its duplicating a check that already exists.
> Yes, absolutely. Enabled slub_debug (or KASAN with its quarantine) can detect
> more double-free errors. But it introduces much bigger performance penalty and
> it's disabled by default.
>> However, this one is always on.
> Yes, I would propose to have this relatively cheap check enabled by default. I
> think it will block a good share of double-free errors. Currently it's really
> easy to turn such a double-free into use-after-free and exploit it, since, as I
> wrote, next two kmalloc() calls return the same address. So we could make
> exploiting harder for a relatively low price.
> Christopher, if I change BUG_ON() to VM_BUG_ON(), it will be disabled by default
> again, right?

Let's merge this with the proposed CONFIG_FREELIST_HARDENED, then the
performance change is behind a config, and we gain the rest of the
freelist protections at the same time:


Kees Cook
Pixel Security

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