linux-kernel - Re: [PATCH] fs: create and use seq_show

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Date:	Mon, 10 Aug 2015 17:12:18 -0400
From:	Paul Moore <paul@...l-moore.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Yan, Zheng" <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
	Ilya Dryomov <idryomov@...il.com>,
	Steve French <sfrench@...ba.org>, Jan Kara <jack@...e.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Theodore Ts'o" <tytso@....edu>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Bob Peterson <rpeterso@...hat.com>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Dave Chinner <david@...morbit.com>, xfs@....sgi.com,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Eric Paris <eparis@...isplace.org>,
	James Morris <james.l.morris@...cle.com>,
	"Serge E. Hallyn" <serge@...lyn.com>, Jens Axboe <axboe@...com>,
	Fabian Frederick <fabf@...net.be>,
	Christoph Hellwig <hch@....de>, Firo Yang <firogm@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jiri Slaby <jslaby@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	Joe Perches <joe@...ches.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: create and use seq_show_option for escaping

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 <keescook@...omium.org>
> Cc: stable@...r.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@...l-moore.com>

-- 
paul moore
www.paul-moore.com

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