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

[Alexander Popov]

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?

Best regards,
Alexander