Re: [PATCH RFC 1/2] mm: Extract SLAB_QUARANTINE from KASAN

[Alexander Popov <alex.popov@linux.com>]

On 15.08.2020 21:54, Matthew Wilcox wrote:
> On Thu, Aug 13, 2020 at 06:19:21PM +0300, Alexander Popov wrote:
>> +config SLAB_QUARANTINE
>> +	bool "Enable slab freelist quarantine"
>> +	depends on !KASAN && (SLAB || SLUB)
>> +	help
>> +	  Enable slab freelist quarantine to break heap spraying technique
>> +	  used for exploiting use-after-free vulnerabilities in the kernel
>> +	  code. If this feature is enabled, freed allocations are stored
>> +	  in the quarantine and can't be instantly reallocated and
>> +	  overwritten by the exploit performing heap spraying.
>> +	  This feature is a part of KASAN functionality.
> 
> After this patch, it isn't part of KASAN any more ;-)

Ok, I'll change that to "this feature is used by KASAN" :)

> The way this is written is a bit too low level.  Let's write it in terms
> that people who don't know the guts of the slab allocator or security
> terminology can understand:
> 
> 	  Delay reuse of freed slab objects.  This makes some security
> 	  exploits harder to execute.  It reduces performance slightly
> 	  as objects will be cache cold by the time they are reallocated,
> 	  and it costs a small amount of memory.
> 
> (feel free to edit this)

Ok, I see.
I'll start from high-level description and add low-level details at the end.

>> +struct qlist_node {
>> +	struct qlist_node *next;
>> +};
> 
> I appreciate this isn't new, but why do we have a new singly-linked-list
> abstraction being defined in this code?

I don't know for sure.
I suppose it is caused by SLAB/SLUB freelist implementation details (qlist_node
in kasan_free_meta is also used for the allocator freelist).

Best regards,
Alexander