linux-kernel - Re: unprivileged mounts git tree

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Date:	Thu, 7 Aug 2008 19:25:37 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	hch@...radead.org, viro@...IV.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: unprivileged mounts git tree

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serue@...ibm.com> writes:
> 
> > Quoting Miklos Szeredi (miklos@...redi.hu):
> >> Here's a git tree of the unprivileged mounts patchset:
> >> 
> >> git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git
> > unprivileged-mounts
> >> 
> >> Could this be added to -mm (and dropped if it's in the way of
> >> something) for some testing and added visibility until it's reviewed
> >> by Christoph/Al?
> >> 
> >> I'm not reposting the whole patchset, since it's essentially the same
> >> as the last submission, only updated to the latest git.  But if
> >> somebody wants it I can post them.
> >> 
> >> Thanks,
> >> Miklos
> >> 
> >> 
> >>  Documentation/filesystems/fuse.txt |   88 ++++++++-
> >>  Documentation/filesystems/proc.txt |   40 ++++
> >>  fs/filesystems.c                   |   60 ++++++
> >>  fs/fuse/inode.c                    |   21 ++
> >>  fs/internal.h                      |    3 +-
> >> fs/namespace.c | 366 +++++++++++++++++++++++++++---------
> >>  fs/pnode.c                         |   22 ++-
> >>  fs/pnode.h                         |    2 +
> >>  fs/super.c                         |   26 ---
> >>  include/linux/fs.h                 |    7 +
> >>  include/linux/mount.h              |    4 +
> >>  kernel/sysctl.c                    |   16 ++
> >>  12 files changed, 527 insertions(+), 128 deletions(-)
> >> 
> >> Miklos Szeredi (10):
> >>       unprivileged mounts: add user mounts to the kernel
> >>       unprivileged mounts: allow unprivileged umount
> >>       unprivileged mounts: propagate error values from clone_mnt
> >>       unprivileged mounts: account user mounts
> >>       unprivileged mounts: allow unprivileged bind mounts
> >>       unprivileged mounts: allow unprivileged mounts
> >>       unprivileged mounts: add sysctl tunable for "safe" property
> >>       unprivileged mounts: make fuse safe
> >>       unprivileged mounts: propagation: inherit owner from parent
> >>       unprivileged mounts: add "no submounts" flag
> >
> > Hi Miklos,
> >
> > so on the bright side I pulled this tree today and it compiled and
> > passed ltp with no problems.
> >
> > But then I played around a bit and found I could do the following:
> >
> > (hmm, i'm trying to remember the exact order :)
> >
> > as root:
> > 	mmount --bind -o user=500 /home/hallyn/etc/ /home/hallyn/etc/
> > 	mount --bind /mnt /mnt
> > 	mount --make-rshared /mnt
> > 	mount --bind /dev /mnt/dev
> >
> > as hallyn:
> > 	mmount --bind /mnt /home/hallyn/etc/mnt
> > 	/usr/src/mmount-0.3/mmount --bind mnt/dev mnt/src
> 
> You are using relative directory names here which makes it confusing.
> I'm assuming you in /home/hallyn/etc ?

Sorry, yeah.

> > Now /mnt/src contained /dev.
> >
> > Is this what we want?
> 
> I don't think so.
> 
> I think the simplest answer is to not allow mounting of shared
> subtrees controlled by a different user.
> 
> Serge I think you are right downgrading the mount from shared to slave
> looks like the sane thing to do if the mount owners match.

I assume you mean "if the mount owners don't match"?

Miklos, what do you think?

The next question then becomes, how can we prove to ourselves that that
closes the last security hole with unprivileged mounts?  So long as
we treat each mount event as a piece of information and look at it as an
information flow problem, maybe we can actually come up with a good
description of the logic that is implemented and show that there is no
way a user can "leak" info...  (where a leak is a mount event, a
violation of intended DAC on open(file) or mkdir, etc)

-serge
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