Re: [PATCH v3 1/1] Randomized slab caches for kmalloc()

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 16, 2023 at 07:18:43PM +0800, GONG, Ruiqi wrote:
> When exploiting memory vulnerabilities, "heap spraying" is a common
> technique targeting those related to dynamic memory allocation (i.e. the
> "heap"), and it plays an important role in a successful exploitation.
> Basically, it is to overwrite the memory area of vulnerable object by
> triggering allocation in other subsystems or modules and therefore
> getting a reference to the targeted memory location. It's usable on
> various types of vulnerablity including use after free (UAF), heap out-
> of-bound write and etc.
> 
> There are (at least) two reasons why the heap can be sprayed: 1) generic
> slab caches are shared among different subsystems and modules, and
> 2) dedicated slab caches could be merged with the generic ones.
> Currently these two factors cannot be prevented at a low cost: the first
> one is a widely used memory allocation mechanism, and shutting down slab
> merging completely via `slub_nomerge` would be overkill.
> 
> To efficiently prevent heap spraying, we propose the following approach:
> to create multiple copies of generic slab caches that will never be
> merged, and random one of them will be used at allocation. The random
> selection is based on the address of code that calls `kmalloc()`, which
> means it is static at runtime (rather than dynamically determined at
> each time of allocation, which could be bypassed by repeatedly spraying
> in brute force). In other words, the randomness of cache selection will
> be with respect to the code address rather than time, i.e. allocations
> in different code paths would most likely pick different caches,
> although kmalloc() at each place would use the same cache copy whenever
> it is executed. In this way, the vulnerable object and memory allocated
> in other subsystems and modules will (most probably) be on different
> slab caches, which prevents the object from being sprayed.
> 
> Meanwhile, the static random selection is further enhanced with a
> per-boot random seed, which prevents the attacker from finding a usable
> kmalloc that happens to pick the same cache with the vulnerable
> subsystem/module by analyzing the open source code.
> 
> The overhead of performance has been tested on a 40-core x86 server by
> comparing the results of `perf bench all` between the kernels with and
> without this patch based on the latest linux-next kernel, which shows
> minor difference. A subset of benchmarks are listed below:
> 
>                 sched/  sched/  syscall/       mem/       mem/
>              messaging    pipe     basic     memcpy     memset
>                  (sec)   (sec)     (sec)   (GB/sec)   (GB/sec)
> 
> control1         0.019   5.459     0.733  15.258789  51.398026
> control2         0.019   5.439     0.730  16.009221  48.828125
> control3         0.019   5.282     0.735  16.009221  48.828125
> control_avg      0.019   5.393     0.733  15.759077  49.684759
> 
> experiment1      0.019   5.374     0.741  15.500992  46.502976
> experiment2      0.019   5.440     0.746  16.276042  51.398026
> experiment3      0.019   5.242     0.752  15.258789  51.398026
> experiment_avg   0.019   5.352     0.746  15.678608  49.766343
> 
> The overhead of memory usage was measured by executing `free` after boot
> on a QEMU VM with 1GB total memory, and as expected, it's positively
> correlated with # of cache copies:
> 
>            control  4 copies  8 copies  16 copies
> 
> total       969.8M    968.2M    968.2M     968.2M
> used         20.0M     21.9M     24.1M      26.7M
> free        936.9M    933.6M    931.4M     928.6M
> available   932.2M    928.8M    926.6M     923.9M
> 
> Signed-off-by: GONG, Ruiqi <gongruiqi@huaweicloud.com>
> Co-developed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>

I think this looks really good. Thanks for the respin! Some
nits/comments/questions below, but I think this can land and get
incrementally improved. Please consider it:

Reviewed-by: Kees Cook <keescook@chromium.org>

> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 791f7453a04f..b7a5387f0dad 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -19,6 +19,9 @@
>  #include <linux/workqueue.h>
>  #include <linux/percpu-refcount.h>
>  
> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
> +#include <linux/hash.h>
> +#endif

I think this can just be included unconditionally, yes?

> [...]
> +extern unsigned long random_kmalloc_seed;
> +
> +static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags, unsigned long caller)
>  {
>  	/*
>  	 * The most common case is KMALLOC_NORMAL, so test for it
>  	 * with a single branch for all the relevant flags.
>  	 */
>  	if (likely((flags & KMALLOC_NOT_NORMAL_BITS) == 0))
> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
> +		return KMALLOC_RANDOM_START + hash_64(caller ^ random_kmalloc_seed,
> +						      CONFIG_RANDOM_KMALLOC_CACHES_BITS);
> +#else
>  		return KMALLOC_NORMAL;
> +#endif

The commit log talks about having no runtime lookup, but that's not
entirely true, given this routine. And xor and a hash_64... I wonder how
expensive this is compared to some kind of constant expression that
could be computed at build time... (the xor should stay, but that's
"cheap").

>  
>  	/*
>  	 * At least one of the flags has to be set. Their priorities in
> @@ -577,7 +589,7 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
>  
>  		index = kmalloc_index(size);
>  		return kmalloc_trace(
> -				kmalloc_caches[kmalloc_type(flags)][index],
> +				kmalloc_caches[kmalloc_type(flags, _RET_IP_)][index],
>  				flags, size);
>  	}
>  	return __kmalloc(size, flags);
> @@ -593,7 +605,7 @@ static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t fla
>  
>  		index = kmalloc_index(size);
>  		return kmalloc_node_trace(
> -				kmalloc_caches[kmalloc_type(flags)][index],
> +				kmalloc_caches[kmalloc_type(flags, _RET_IP_)][index],
>  				flags, node, size);
>  	}
>  	return __kmalloc_node(size, flags, node);

The use of _RET_IP_ is generally fine here, but I wonder about some of
the allocation wrappers (like devm_kmalloc(), etc). I think those aren't
being bucketed correctly? Have you checked that?

> [...]
> @@ -776,12 +781,44 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
>  #define KMALLOC_RCL_NAME(sz)
>  #endif
>  
> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
> +#define __KMALLOC_RANDOM_CONCAT(a, b) a ## b
> +#define KMALLOC_RANDOM_NAME(N, sz) __KMALLOC_RANDOM_CONCAT(KMA_RAND_, N)(sz)
> +#if CONFIG_RANDOM_KMALLOC_CACHES_BITS >= 1
> +#define KMA_RAND_1(sz)                  .name[KMALLOC_RANDOM_START +  0] = "kmalloc-random-01-" #sz,

I wonder if this name is getting too long? Should "random" be "rnd" ?
*shrug*

> [...]
> +#define KMA_RAND_16(sz) KMA_RAND_15(sz) .name[KMALLOC_RANDOM_START + 15] = "kmalloc-random-16-" #sz,

And if we wanted to save another character, this could be numbered 0-f,
but I defer these aesthetics to Vlastimil. :)

-Kees

-- 
Kees Cook