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

[Alexander Popov]

On 15.08.2020 19:52, Kees Cook wrote:
> On Thu, Aug 13, 2020 at 06:19:21PM +0300, Alexander Popov wrote:
>> Heap spraying is an exploitation technique that aims to put controlled
>> bytes at a predetermined memory location on the heap. Heap spraying for
>> exploiting use-after-free in the Linux kernel relies on the fact that on
>> kmalloc(), the slab allocator returns the address of the memory that was
>> recently freed. Allocating a kernel object with the same size and
>> controlled contents allows overwriting the vulnerable freed object.
>>
>> Let's extract slab freelist quarantine from KASAN functionality and
>> call it CONFIG_SLAB_QUARANTINE. This feature breaks widespread 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.
> 
> It may be worth clarifying that this is specifically only direct UAF and
> doesn't help with spray-and-overflow-into-a-neighboring-object attacks
> (i.e. both tend to use sprays, but the former doesn't depend on a write
> overflow).

Right, thank you.

>> Signed-off-by: Alexander Popov <alex.popov@xxxxxxxxx>
>> ---
>>  include/linux/kasan.h      | 107 ++++++++++++++++++++-----------------
>>  include/linux/slab_def.h   |   2 +-
>>  include/linux/slub_def.h   |   2 +-
>>  init/Kconfig               |  11 ++++
>>  mm/Makefile                |   3 +-
>>  mm/kasan/Makefile          |   2 +
>>  mm/kasan/kasan.h           |  75 +++++++++++++-------------
>>  mm/kasan/quarantine.c      |   2 +
>>  mm/kasan/slab_quarantine.c |  99 ++++++++++++++++++++++++++++++++++
>>  mm/slub.c                  |   2 +-
>>  10 files changed, 216 insertions(+), 89 deletions(-)
>>  create mode 100644 mm/kasan/slab_quarantine.c
>>
>> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
>> index 087fba34b209..b837216f760c 100644
>> --- a/include/linux/kasan.h
>> +++ b/include/linux/kasan.h

[...]

>>  #else /* CONFIG_KASAN_GENERIC */
>> +static inline void kasan_record_aux_stack(void *ptr) {}
>> +#endif /* CONFIG_KASAN_GENERIC */
>>  
>> +#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_SLAB_QUARANTINE)
>> +void kasan_cache_shrink(struct kmem_cache *cache);
>> +void kasan_cache_shutdown(struct kmem_cache *cache);
>> +#else /* CONFIG_KASAN_GENERIC || CONFIG_SLAB_QUARANTINE */
>>  static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
>>  static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
>> -static inline void kasan_record_aux_stack(void *ptr) {}
>> -
>> -#endif /* CONFIG_KASAN_GENERIC */
>> +#endif /* CONFIG_KASAN_GENERIC || CONFIG_SLAB_QUARANTINE */
> 
> In doing this extraction, I wonder if function naming should be changed?
> If it's going to live a new life outside of KASAN proper, maybe call
> these functions quarantine_cache_*()? But perhaps that's too much
> churn...

These functions are kasan handlers that are called by allocator.
I.e. allocator calls kasan handlers, and then kasan handlers call
quarantine_put(), quarantine_reduce() and quarantine_remove_cache() among other
things.

Andrey Konovalov wrote:
> If quarantine is to be used without the rest of KASAN, I'd prefer for
> it to be separated from KASAN completely: move to e.g. mm/quarantine.c
> and don't mention KASAN in function/config names.

Hmm, making quarantine completely separate from KASAN would bring troubles.

Currently, in many special places the allocator calls KASAN handlers:
  kasan_cache_create()
  kasan_slab_free()
  kasan_kmalloc_large()
  kasan_krealloc()
  kasan_slab_alloc()
  kasan_kmalloc()
  kasan_cache_shrink()
  kasan_cache_shutdown()
  and some others.
These functions do a lot of interesting things and also work with the quarantine
using these helpers:
  quarantine_put()
  quarantine_reduce()
  quarantine_remove_cache()

Making quarantine completely separate from KASAN would require to move some
internal logic of these KASAN handlers to allocator code.

In this patch I used another approach, that doesn't require changing the API
between allocators and KASAN. I added linux/mm/kasan/slab_quarantine.c with slim
KASAN handlers that implement the minimal functionality needed for quarantine.

Do you think that it's a bad solution?

>>  #ifdef CONFIG_KASAN_SW_TAGS
>>  
>> diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
>> index 9eb430c163c2..fc7548f27512 100644
>> --- a/include/linux/slab_def.h
>> +++ b/include/linux/slab_def.h
>> @@ -72,7 +72,7 @@ struct kmem_cache {
>>  	int obj_offset;
>>  #endif /* CONFIG_DEBUG_SLAB */
>>  
>> -#ifdef CONFIG_KASAN
>> +#if defined(CONFIG_KASAN) || defined(CONFIG_SLAB_QUARANTINE)
>>  	struct kasan_cache kasan_info;
>>  #endif
>>  
>> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
>> index 1be0ed5befa1..71020cee9fd2 100644
>> --- a/include/linux/slub_def.h
>> +++ b/include/linux/slub_def.h
>> @@ -124,7 +124,7 @@ struct kmem_cache {
>>  	unsigned int *random_seq;
>>  #endif
>>  
>> -#ifdef CONFIG_KASAN
>> +#if defined(CONFIG_KASAN) || defined(CONFIG_SLAB_QUARANTINE)
>>  	struct kasan_cache kasan_info;
>>  #endif
>>  
>> diff --git a/init/Kconfig b/init/Kconfig
>> index d6a0b31b13dc..de5aa061762f 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1931,6 +1931,17 @@ config SLAB_FREELIST_HARDENED
>>  	  sanity-checking than others. This option is most effective with
>>  	  CONFIG_SLUB.
>>  
>> +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.
>> +
> 
> To make this available to distros, I think this needs to be more than
> just a CONFIG. I'd love to see this CONFIG control the availability, but
> have a boot param control a ro-after-init static branch for these
> functions (like is done for init_on_alloc, hardened usercopy, etc). Then
> the branch can be off by default for regular distro users, and more
> cautious folks could enable it with a boot param without having to roll
> their own kernels.

Good point, thanks, added to TODO list.

>> [...]
>> +struct kasan_track {
>> +	u32 pid;
> 
> pid_t?

Ok, I can change it (here I only moved the current definition of kasan_track).

>> +	depot_stack_handle_t stack;
>> +};
>> [...]
>> +#if defined(CONFIG_KASAN_GENERIC) && \
>> +	(defined(CONFIG_SLAB) || defined(CONFIG_SLUB)) || \
>> +	defined(CONFIG_SLAB_QUARANTINE)
> 
> This seems a bit messy. Perhaps an invisible CONFIG to do this logic and
> then the files can test for that? CONFIG_USE_SLAB_QUARANTINE or
> something?

Ok, thanks, I'll try that.

>> [...]
>> + * Heap spraying is an exploitation technique that aims to put controlled
>> + * bytes at a predetermined memory location on the heap. Heap spraying for
>> + * exploiting use-after-free in the Linux kernel relies on the fact that on
>> + * kmalloc(), the slab allocator returns the address of the memory that was
>> + * recently freed. Allocating a kernel object with the same size and
>> + * controlled contents allows overwriting the vulnerable freed object.
>> + *
>> + * If freed allocations are stored in the quarantine, they can't be
>> + * instantly reallocated and overwritten by the exploit performing
>> + * heap spraying.
> 
> I would clarify this with the details of what is actually happening:

Ok.

> the allocation isn't _moved_ to a quarantine, yes? It's only marked as not
> available for allocation?

The allocation is put into the quarantine queues, where all allocations wait for
actual freeing.

>> + */
>> +
>> +#include <linux/kasan.h>
>> +#include <linux/bug.h>
>> +#include <linux/slab.h>
>> +#include <linux/mm.h>
>> +#include "../slab.h"
>> +#include "kasan.h"
>> +
>> +void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>> +			slab_flags_t *flags)
>> +{
>> +	cache->kasan_info.alloc_meta_offset = 0;
>> +
>> +	if (cache->flags & SLAB_TYPESAFE_BY_RCU || cache->ctor ||
>> +	     cache->object_size < sizeof(struct kasan_free_meta)) {
>> +		cache->kasan_info.free_meta_offset = *size;
>> +		*size += sizeof(struct kasan_free_meta);
>> +		BUG_ON(*size > KMALLOC_MAX_SIZE);
> 
> Please don't use BUG_ON()[1].

Ok!

> Interesting!
> 
> -Kees
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on
>