linux-kernel - Re: unprivileged mounts git tree

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Message-Id: <E1Kb8Y0-0008Gh-FP@pomaz-ex.szeredi.hu>
Date:	Thu, 04 Sep 2008 08:42:28 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	miklos@...redi.hu, 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

On Wed, 3 Sep 2008, Serge E. Hallyn wrote:
> Quoting Miklos Szeredi (miklos@...redi.hu):
> > On Wed, 3 Sep 2008, Serge E. Hallyn wrote:
> > > Ooh.
> > > 
> > > You predicate the turning of shared mount to a slave mount on
> > > !capable(CAP_SYS_ADMIN).  But in fact it's the mount by a privileged
> > > user, turning the mount into a user mount, which you want to convert.
> > > So my series of steps was:
> > > 
> > > 	as root:
> > > 		(1) mount --bind /mnt /mnt
> > > 		(2) mount --make-rshared /mnt
> > > 		(3) /usr/src/mmount-0.3/mmount --bind -o user=hallyn /mnt \
> > > 			/home/hallyn/etc/mnt
> > > 	as hallyn:
> > > 		(4) mount --bind /usr /home/hallyn/etc/mnt/usr
> > > 
> > > You are turning mounts from shared->slave at step 4, but in fact we need
> > > to do it at step 3, where we do have CAP_SYS_ADMIN.
> > 
> > Well, that's arguable: I think root should be able to shoot itself in
> > the foot by doing step 3.
> 
> Maybe I'm not thinking right, but long-term is there any reason why we
> should require privilege in order to do step 3, so long as the user has
> read access to the source and write access to the destination?
> 
> I don't think there is.  Other than this glitch.  That's a powerful
> reason to fix the glitch.

Agreed, without privileges it's unacceptable to allow step 3 as is.

> The other argument is that, frankly, I think most people are still
> either unaware of, or confused by, mounts propagation.  Letting root
> shoot himself in the foot is reasonable only to a point.

Hmm, I think there are infinite ways in which root can mess up mount
propagation, and this is not even the worst.  I'm not trying to
belittle this bug: done unprivileged it's unacceptable.  But with
privileges, I really don't know if we should change the propagation
semantics for this corner case, they are complicated enough already.

> > Generally we don't restrict what root can
> > do.  OTOH I agree that current behavior is ugly in that it provides
> > different semantics for privileged/non-privileged callers.
> > 
> > Perhaps it would be cleaner to simply not allow step 4, instead of
> > playing tricks with changing the propagation type.
> 
> If the user or admin can simply (I haven't tested)
> 
> 	mmount --bind --make-rslave -o user=hallyn /mnt \
> 		/home/hallyn/etc/mnt
> 
> then returning -EPERM if --make-rslave was not provided is reasonable
> IMO.

Right, that sounds perfect.  the only problem is, bind mount currently
ignores the propagation flags, for no good reason I can see.

That's a separate patch though.  I'll look into it.

Thanks,
Miklos
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