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From: Alexander Popov <>
To: Kees Cook <>
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
Date: Tue, 18 Jul 2017 22:56:23 +0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 17.07.2017 22:11, Kees Cook wrote:
> 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:

Hello Kees,

If I change BUG_ON() to VM_BUG_ON(), this check will work at least on Fedora
since it has CONFIG_DEBUG_VM enabled. Debian based distros have this option
disabled. Do you like that more than having this check under

If you insist on putting this check under CONFIG_FREELIST_HARDENED, should I
rebase onto your patch and send again?

Best regards,

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  reply	other threads:[~2017-07-18 19:56 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-17 16:45 [PATCH 1/1] mm/slub.c: add a naive detection of double free or corruption Alexander Popov
2017-07-17 16:57 ` Christopher Lameter
2017-07-17 17:54 ` Matthew Wilcox
2017-07-17 18:04   ` Christopher Lameter
2017-07-17 19:01     ` Alexander Popov
2017-07-17 19:11       ` Kees Cook
2017-07-18 19:56         ` Alexander Popov [this message]
2017-07-18 20:04           ` Kees Cook
2017-07-19  8:38             ` Alexander Popov
2017-07-19 14:02             ` Christopher Lameter
2017-07-18 14:57       ` Christopher Lameter
2017-07-17 18:23   ` Alexander Popov

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