On Friday, August 07, 2015 04:41:50 PM Kees Cook wrote:

Many file systems that implement the show_options hook fail to correctly
escape their output which could lead to unescaped characters (e.g. new
lines) leaking into /proc/mounts and /proc/[pid]/mountinfo files. This
could lead to confusion, spoofed entries (resulting in things like
systemd issuing false d-bus "mount" notifications), and who knows
what else. This looks like it would only be the root user stepping on
themselves, but it's possible weird things could happen in containers
or in other situations with delegated mount privileges.

Here's an example using overlay with setuid fusermount trusting the
contents of /proc/mounts (via the /etc/mtab symlink). Imagine the use of
"sudo" is something more sneaky:

$ BASE="ovl"
$ MNT="$BASE/mnt"
$ LOW="$BASE/lower"
$ UP="$BASE/upper"
$ WORK="$BASE/work/ 0 0
none /proc fuse.pwn user_id=1000"
$ mkdir -p "$LOW" "$UP" "$WORK"
$ sudo mount -t overlay -o "lowerdir=$LOW,upperdir=$UP,workdir=$WORK" none
/mnt $ cat /proc/mounts
none /root/ovl/mnt overlay
rw,relatime,lowerdir=ovl/lower,upperdir=ovl/upper,workdir=ovl/work/ 0 0
none /proc fuse.pwn user_id=1000 0 0
$ fusermount -u /proc
$ cat /proc/mounts
cat: /proc/mounts: No such file or directory

This fixes the problem by adding new seq_show_option and seq_show_option_n
helpers, and updating the vulnerable show_option handlers to use them as
needed. Some, like SELinux, need to be open coded due to unusual existing
escape mechanisms.

Signed-off-by: Kees Cook <redacted>
Cc: stable@vger.kernel.org
---
 fs/ceph/super.c          |  2 +-
 fs/cifs/cifsfs.c         |  6 +++---
 fs/ext3/super.c          |  4 ++--
 fs/ext4/super.c          |  4 ++--
 fs/gfs2/super.c          |  6 +++---
 fs/hfs/super.c           |  4 ++--
 fs/hfsplus/options.c     |  4 ++--
 fs/hostfs/hostfs_kern.c  |  2 +-
 fs/ocfs2/super.c         |  4 ++--
 fs/overlayfs/super.c     |  6 +++---
 fs/reiserfs/super.c      |  8 +++++---
 fs/xfs/xfs_super.c       |  4 ++--
 include/linux/seq_file.h | 34 ++++++++++++++++++++++++++++++++++
 kernel/cgroup.c          |  7 ++++---
 net/ceph/ceph_common.c   |  7 +++++--
 security/selinux/hooks.c |  2 +-
 16 files changed, 72 insertions(+), 32 deletions(-)

The SELinux changes look fine to me.

Acked-by: Paul Moore <paul@paul-moore.com>

-- 
paul moore
www.paul-moore.com

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