Re: [PATCH RFC v2 0/6] Break heap spraying needed for exploiting use-after-free

[Alexander Popov]

Hello! I have some performance numbers. Please see below.

On 29.09.2020 21:35, Alexander Popov wrote:
> Hello everyone! Requesting for your comments.
> 
> This is the second version of the heap quarantine prototype for the Linux
> kernel. I performed a deeper evaluation of its security properties and
> developed new features like quarantine randomization and integration with
> init_on_free. That is fun! See below for more details.
> 
> 
> Rationale
> =========
> 
> Use-after-free vulnerabilities in the Linux kernel are very popular for
> exploitation. There are many examples, some of them:
>  https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html
>  https://googleprojectzero.blogspot.com/2019/11/bad-binder-android-in-wild-exploit.html?m=1
>  https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
> 
> Use-after-free exploits usually employ heap spraying technique.
> Generally it 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. So allocating a kernel object with
> the same size and controlled contents allows overwriting the vulnerable
> freed object.
> 
> I've found an easy way to break the heap spraying for use-after-free
> exploitation. I extracted slab freelist quarantine from KASAN functionality
> and called it CONFIG_SLAB_QUARANTINE. Please see patch 1/6.
> 
> If this feature is enabled, freed allocations are stored in the quarantine
> queue where they wait for actual freeing. So they can't be instantly
> reallocated and overwritten by use-after-free exploits.
> 
> N.B. Heap spraying for out-of-bounds exploitation is another technique,
> heap quarantine doesn't break it.
> 
> 
> Security properties
> ===================
> 
> For researching security properties of the heap quarantine I developed 2 lkdtm
> tests (see the patch 5/6).
> 
> The first test is called lkdtm_HEAP_SPRAY. It allocates and frees an object
> from a separate kmem_cache and then allocates 400000 similar objects.
> I.e. this test performs an original heap spraying technique for use-after-free
> exploitation.
> 
> If CONFIG_SLAB_QUARANTINE is disabled, the freed object is instantly
> reallocated and overwritten:
>   # echo HEAP_SPRAY > /sys/kernel/debug/provoke-crash/DIRECT
>    lkdtm: Performing direct entry HEAP_SPRAY
>    lkdtm: Allocated and freed spray_cache object 000000002b5b3ad4 of size 333
>    lkdtm: Original heap spraying: allocate 400000 objects of size 333...
>    lkdtm: FAIL: attempt 0: freed object is reallocated
> 
> If CONFIG_SLAB_QUARANTINE is enabled, 400000 new allocations don't overwrite
> the freed object:
>   # echo HEAP_SPRAY > /sys/kernel/debug/provoke-crash/DIRECT
>    lkdtm: Performing direct entry HEAP_SPRAY
>    lkdtm: Allocated and freed spray_cache object 000000009909e777 of size 333
>    lkdtm: Original heap spraying: allocate 400000 objects of size 333...
>    lkdtm: OK: original heap spraying hasn't succeed
> 
> That happens because pushing an object through the quarantine requires _both_
> allocating and freeing memory. Objects are released from the quarantine on
> new memory allocations, but only when the quarantine size is over the limit.
> And the quarantine size grows on new memory freeing.
> 
> That's why I created the second test called lkdtm_PUSH_THROUGH_QUARANTINE.
> It allocates and frees an object from a separate kmem_cache and then performs
> kmem_cache_alloc()+kmem_cache_free() for that cache 400000 times.
> This test effectively pushes the object through the heap quarantine and
> reallocates it after it returns back to the allocator freelist:
>   # echo PUSH_THROUGH_QUARANTINE > /sys/kernel/debug/provoke-crash/
>    lkdtm: Performing direct entry PUSH_THROUGH_QUARANTINE
>    lkdtm: Allocated and freed spray_cache object 000000008fdb15c3 of size 333
>    lkdtm: Push through quarantine: allocate and free 400000 objects of size 333...
>    lkdtm: Target object is reallocated at attempt 182994
>   # echo PUSH_THROUGH_QUARANTINE > /sys/kernel/debug/provoke-crash/
>    lkdtm: Performing direct entry PUSH_THROUGH_QUARANTINE
>    lkdtm: Allocated and freed spray_cache object 000000004e223cbe of size 333
>    lkdtm: Push through quarantine: allocate and free 400000 objects of size 333...
>    lkdtm: Target object is reallocated at attempt 186830
>   # echo PUSH_THROUGH_QUARANTINE > /sys/kernel/debug/provoke-crash/
>    lkdtm: Performing direct entry PUSH_THROUGH_QUARANTINE
>    lkdtm: Allocated and freed spray_cache object 000000007663a058 of size 333
>    lkdtm: Push through quarantine: allocate and free 400000 objects of size 333...
>    lkdtm: Target object is reallocated at attempt 182010
> 
> As you can see, the number of the allocations that are needed for overwriting
> the vulnerable object is almost the same. That would be good for stable
> use-after-free exploitation and should not be allowed.
> That's why I developed the quarantine randomization (see the patch 4/6).
> 
> This randomization required very small hackish changes of the heap quarantine
> mechanism. At first all quarantine batches are filled by objects. Then during
> the quarantine reducing I randomly choose and free 1/2 of objects from a
> randomly chosen batch. Now the randomized quarantine releases the freed object
> at an unpredictable moment:
>    lkdtm: Target object is reallocated at attempt 107884
>    lkdtm: Target object is reallocated at attempt 265641
>    lkdtm: Target object is reallocated at attempt 100030
>    lkdtm: Target object is NOT reallocated in 400000 attempts
>    lkdtm: Target object is reallocated at attempt 204731
>    lkdtm: Target object is reallocated at attempt 359333
>    lkdtm: Target object is reallocated at attempt 289349
>    lkdtm: Target object is reallocated at attempt 119893
>    lkdtm: Target object is reallocated at attempt 225202
>    lkdtm: Target object is reallocated at attempt 87343
> 
> However, this randomization alone would not disturb the attacker, because
> the quarantine stores the attacker's data (the payload) in the sprayed objects.
> I.e. the reallocated and overwritten vulnerable object contains the payload
> until the next reallocation (very bad).
> 
> Hence heap objects should be erased before going to the heap quarantine.
> Moreover, filling them by zeros gives a chance to detect use-after-free
> accesses to non-zero data while an object stays in the quarantine (nice!).
> That functionality already exists in the kernel, it's called init_on_free.
> I integrated it with CONFIG_SLAB_QUARANTINE in the patch 3/6.
> 
> During that work I found a bug: in CONFIG_SLAB init_on_free happens too
> late, and heap objects go to the KASAN quarantine being dirty. See the fix
> in the patch 2/6.
> 
> For deeper understanding of the heap quarantine inner workings, I attach
> the patch 6/6, which contains verbose debugging (not for merge).
> It's very helpful, see the output example:
>    quarantine: PUT 508992 to tail batch 123, whole sz 65118872, batch sz 508854
>    quarantine: whole sz exceed max by 494552, REDUCE head batch 0 by 415392, leave 396304
>    quarantine: data level in batches:
>      0 - 77%
>      1 - 108%
>      2 - 83%
>      3 - 21%
>    ...
>      125 - 75%
>      126 - 12%
>      127 - 108%
>    quarantine: whole sz exceed max by 79160, REDUCE head batch 12 by 14160, leave 17608
>    quarantine: whole sz exceed max by 65000, REDUCE head batch 75 by 218328, leave 195232
>    quarantine: PUT 508992 to tail batch 124, whole sz 64979984, batch sz 508854
>    ...
> 
> 
> Changes in v2
> =============
> 
>  - Added heap quarantine randomization (the patch 4/6).
> 
>  - Integrated CONFIG_SLAB_QUARANTINE with init_on_free (the patch 3/6).
> 
>  - Fixed late init_on_free in CONFIG_SLAB (the patch 2/6).
> 
>  - Added lkdtm_PUSH_THROUGH_QUARANTINE test.
> 
>  - Added the quarantine verbose debugging (the patch 6/6, not for merge).
> 
>  - Improved the descriptions according to the feedback from Kees Cook
>    and Matthew Wilcox.
> 
>  - Made fixes recommended by Kees Cook:
> 
>    * Avoided BUG_ON() in kasan_cache_create() by handling the error and
>      reporting with WARN_ON().
> 
>    * Created a separate kmem_cache for new lkdtm tests.
> 
>    * Fixed kasan_track.pid type to pid_t.
> 
> 
> TODO for the next prototypes
> ============================
> 
> 1. Performance evaluation and optimization.
>    I would really appreciate your ideas about performance testing of a
>    kernel with the heap quarantine. The first prototype was tested with
>    hackbench and kernel build timing (which showed very different numbers).
>    Earlier the developers similarly tested init_on_free functionality.
>    However, Brad Spengler says in his twitter that such testing method
>    is poor.

I've made various tests on real hardware and in virtual machines:
 1) network throughput test using iperf
     server: iperf -s -f K
     client: iperf -c 127.0.0.1 -t 60 -f K
 2) scheduler stress test
     hackbench -s 4000 -l 500 -g 15 -f 25 -P
 3) building the defconfig kernel
     time make -j2

I compared Linux kernel 5.9.0-rc6 with:
 - init_on_free=off,
 - init_on_free=on,
 - CONFIG_SLAB_QUARANTINE=y (which enables init_on_free).

Each test was performed 5 times. I will show the mean values.
If you are interested, I can share all the results and calculate standard deviation.

Real hardware, Intel Core i7-6500U CPU
 1) Network throughput test with iperf
     init_on_free=off: 5467152.2 KBytes/sec
     init_on_free=on: 3937545 KBytes/sec (-28.0% vs init_on_free=off)
     CONFIG_SLAB_QUARANTINE: 3858848.6 KBytes/sec (-2.0% vs init_on_free=on)
 2) Scheduler stress test with hackbench
     init_on_free=off: 8.5364s
     init_on_free=on: 8.9858s (+5.3% vs init_on_free=off)
     CONFIG_SLAB_QUARANTINE: 17.2232s (+91.7% vs init_on_free=on)
 3) Building the defconfig kernel:
     init_on_free=off: 10m54.475s
     init_on_free=on: 11m5.745s (+1.7% vs init_on_free=off)
     CONFIG_SLAB_QUARANTINE: 11m13.291s (+1.1% vs init_on_free=on)

Virtual machine, QEMU/KVM
 1) Network throughput test with iperf
     init_on_free=off: 3554237.4 KBytes/sec
     init_on_free=on: 2828887.4 KBytes/sec (-20.4% vs init_on_free=off)
     CONFIG_SLAB_QUARANTINE: 2587308.2 KBytes/sec (-8.5% vs init_on_free=on)
 2) Scheduler stress test with hackbench
     init_on_free=off: 19.3602s
     init_on_free=on: 20.8854s (+7.9% vs init_on_free=off)
     CONFIG_SLAB_QUARANTINE: 30.0746s (+44.0% vs init_on_free=on)

We can see that the results of these tests are quite diverse.
Your interpretation of the results and ideas of other tests are welcome.

N.B. There was NO performance optimization made for this version of the heap
quarantine prototype. The main effort was put into researching its security
properties (hope for your feedback). Performance optimization will be done in
further steps, if we see that my work is worth doing.

> 2. Complete separation of CONFIG_SLAB_QUARANTINE from KASAN (feedback
>    from Andrey Konovalov).
> 
> 3. Adding a kernel boot parameter for enabling/disabling the heap quaranitne
>    (feedback from Kees Cook).
> 
> 4. Testing the heap quarantine in near-OOM situations (feedback from
>    Pavel Machek).
> 
> 5. Does this work somehow help or disturb the integration of the
>    Memory Tagging for the Linux kernel?
> 
> 6. After rebasing the series onto v5.9.0-rc6, CONFIG_SLAB kernel started to
>    show warnings about few slab caches that have no space for additional
>    metadata. It needs more investigation. I believe it affects KASAN bug
>    detection abilities as well. Warning example:
>      WARNING: CPU: 0 PID: 0 at mm/kasan/slab_quarantine.c:38 kasan_cache_create+0x37/0x50
>      Modules linked in:
>      CPU: 0 PID: 0 Comm: swapper Not tainted 5.9.0-rc6+ #1
>      Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014
>      RIP: 0010:kasan_cache_create+0x37/0x50
>      ...
>      Call Trace:
>       __kmem_cache_create+0x74/0x250
>       create_boot_cache+0x6d/0x91
>       create_kmalloc_cache+0x57/0x93
>       new_kmalloc_cache+0x39/0x47
>       create_kmalloc_caches+0x33/0xd9
>       start_kernel+0x25b/0x532
>       secondary_startup_64+0xb6/0xc0
> 
> Thanks in advance for your feedback.
> Best regards,
> Alexander
> 
> 
> Alexander Popov (6):
>   mm: Extract SLAB_QUARANTINE from KASAN
>   mm/slab: Perform init_on_free earlier
>   mm: Integrate SLAB_QUARANTINE with init_on_free
>   mm: Implement slab quarantine randomization
>   lkdtm: Add heap quarantine tests
>   mm: Add heap quarantine verbose debugging (not for merge)
> 
>  drivers/misc/lkdtm/core.c  |   2 +
>  drivers/misc/lkdtm/heap.c  | 110 +++++++++++++++++++++++++++++++++++++
>  drivers/misc/lkdtm/lkdtm.h |   2 +
>  include/linux/kasan.h      | 107 ++++++++++++++++++++----------------
>  include/linux/slab_def.h   |   2 +-
>  include/linux/slub_def.h   |   2 +-
>  init/Kconfig               |  14 +++++
>  mm/Makefile                |   3 +-
>  mm/kasan/Makefile          |   2 +
>  mm/kasan/kasan.h           |  75 +++++++++++++------------
>  mm/kasan/quarantine.c      | 102 ++++++++++++++++++++++++++++++----
>  mm/kasan/slab_quarantine.c | 106 +++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c            |  22 ++++++++
>  mm/slab.c                  |   5 +-
>  mm/slub.c                  |   2 +-
>  15 files changed, 455 insertions(+), 101 deletions(-)
>  create mode 100644 mm/kasan/slab_quarantine.c
>