linux-kernel - Re: unprivileged mounts git tree

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Message-ID: <20080904233240.GB9995@us.ibm.com>
Date:	Thu, 4 Sep 2008 18:32:40 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	ebiederm@...ssion.com, 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 Miklos Szeredi (miklos@...redi.hu):
> On Thu, 4 Sep 2008, Serge E. Hallyn wrote:
> > We still have the original problem.
> > 
> > When root does
> > 
> > 	mount -bind /mnt /mnt
> > 	mount --make-rshared /mnt
> > 	mount --bind -o user=hallyn /mnt /home/hallyn/mnt
> > 
> > and hallyn does
> > 
> > 	mount --bind /usr /home/hallyn/mnt/usr
> > 
> > then the kernel happily propagates the mount to /mnt/usr.
> 
> Obviously, and that's exactly what root _instructed_ in the last step.
> If it's a security problem, root shouldn't do that.
> 
> Your original bug report correctly pointed out the real security
> problem:
> 
> |  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
> 
> Here root does nothing "unsafe", yet the user can get propagation back
> into /mnt, due to the fact that a bind mount makes the new mount part
> of the old peer group.  This is the security hole that is fixed, and
> AFAICS the only security hole related to propagation vs. user mounts.
> 
> (I'm going to be offline tomorrow and the weekend, but will hopefully
> have email access next week).
> 
> Thanks,
> Miklos

(&(*$&%, you're right, of course.

Ok, will play with it a bit more, but I think it'd be *great* to see
this show up in -mm again.

-serge
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