Re: [PATCH 1/1] mm/slub.c: add a naive detection of double free or corruption

[Alexander Popov]

On 17.07.2017 22:11, Kees Cook wrote:
> On Mon, Jul 17, 2017 at 12:01 PM, Alexander Popov <alex.popov@xxxxxxxxx> 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:
> 
> http://www.openwall.com/lists/kernel-hardening/2017/07/06/1

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
CONFIG_FREELIST_HARDENED?

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

Best regards,
Alexander