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Date:   Sat, 15 Aug 2020 09:59:56 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Alexander Popov <alex.popov@linux.com>
Cc:     Jann Horn <jannh@google.com>, Will Deacon <will@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
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Subject: Re: [PATCH RFC 2/2] lkdtm: Add heap spraying test
Message-ID: <202008150952.E81C4A52F@keescook>
References: <20200813151922.1093791-1-alex.popov@linux.com>
 <20200813151922.1093791-3-alex.popov@linux.com>
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On Thu, Aug 13, 2020 at 06:19:22PM +0300, Alexander Popov wrote:
> Add a simple test for CONFIG_SLAB_QUARANTINE.
> 
> It performs heap spraying that aims to reallocate the recently freed heap
> object. This technique is used for exploiting use-after-free
> vulnerabilities in the kernel code.
> 
> This test shows that CONFIG_SLAB_QUARANTINE breaks heap spraying
> exploitation technique.

Yay tests!

> 
> Signed-off-by: Alexander Popov <alex.popov@linux.com>
> ---
>  drivers/misc/lkdtm/core.c  |  1 +
>  drivers/misc/lkdtm/heap.c  | 40 ++++++++++++++++++++++++++++++++++++++
>  drivers/misc/lkdtm/lkdtm.h |  1 +
>  3 files changed, 42 insertions(+)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index a5e344df9166..78b7669c35eb 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -126,6 +126,7 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(SLAB_FREE_DOUBLE),
>  	CRASHTYPE(SLAB_FREE_CROSS),
>  	CRASHTYPE(SLAB_FREE_PAGE),
> +	CRASHTYPE(HEAP_SPRAY),
>  	CRASHTYPE(SOFTLOCKUP),
>  	CRASHTYPE(HARDLOCKUP),
>  	CRASHTYPE(SPINLOCKUP),
> diff --git a/drivers/misc/lkdtm/heap.c b/drivers/misc/lkdtm/heap.c
> index 1323bc16f113..a72a241e314a 100644
> --- a/drivers/misc/lkdtm/heap.c
> +++ b/drivers/misc/lkdtm/heap.c
> @@ -205,6 +205,46 @@ static void ctor_a(void *region)
>  static void ctor_b(void *region)
>  { }
>  
> +#define HEAP_SPRAY_SIZE 128
> +
> +void lkdtm_HEAP_SPRAY(void)
> +{
> +	int *addr;
> +	int *spray_addrs[HEAP_SPRAY_SIZE] = { 0 };

(the 0 isn't needed -- and it was left there, it should be NULL)

> +	unsigned long i = 0;
> +
> +	addr = kmem_cache_alloc(a_cache, GFP_KERNEL);

I would prefer this test add its own cache (e.g. spray_cache), to avoid
misbehaviors between tests. (e.g. the a and b caches already run the
risk of getting corrupted weirdly.)

> +	if (!addr) {
> +		pr_info("Unable to allocate memory in lkdtm-heap-a cache\n");
> +		return;
> +	}
> +
> +	*addr = 0x31337;
> +	kmem_cache_free(a_cache, addr);
> +
> +	pr_info("Performing heap spraying...\n");
> +	for (i = 0; i < HEAP_SPRAY_SIZE; i++) {
> +		spray_addrs[i] = kmem_cache_alloc(a_cache, GFP_KERNEL);
> +		*spray_addrs[i] = 0x31337;
> +		pr_info("attempt %lu: spray alloc addr %p vs freed addr %p\n",
> +						i, spray_addrs[i], addr);

That's 128 lines spewed into dmesg... I would leave this out.

> +		if (spray_addrs[i] == addr) {
> +			pr_info("freed addr is reallocated!\n");
> +			break;
> +		}
> +	}
> +
> +	if (i < HEAP_SPRAY_SIZE)
> +		pr_info("FAIL! Heap spraying succeed :(\n");

I'd move this into the "if (spray_addrs[i] == addr)" test instead of the
pr_info() that is there.

> +	else
> +		pr_info("OK! Heap spraying hasn't succeed :)\n");

And then make this an "if (i == HEAP_SPRAY_SIZE)" test

> +
> +	for (i = 0; i < HEAP_SPRAY_SIZE; i++) {
> +		if (spray_addrs[i])
> +			kmem_cache_free(a_cache, spray_addrs[i]);
> +	}
> +}
> +
>  void __init lkdtm_heap_init(void)
>  {
>  	double_free_cache = kmem_cache_create("lkdtm-heap-double_free",
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index 8878538b2c13..dfafb4ae6f3a 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -45,6 +45,7 @@ void lkdtm_READ_BUDDY_AFTER_FREE(void);
>  void lkdtm_SLAB_FREE_DOUBLE(void);
>  void lkdtm_SLAB_FREE_CROSS(void);
>  void lkdtm_SLAB_FREE_PAGE(void);
> +void lkdtm_HEAP_SPRAY(void);
>  
>  /* lkdtm_perms.c */
>  void __init lkdtm_perms_init(void);
> -- 
> 2.26.2
> 

I assume enabling the quarantine defense also ends up being seen in the
SLAB_FREE_DOUBLE LKDTM test too, yes?

-- 
Kees Cook