linux-kernel - Re: [patch 8/9] unprivileged mounts: propagation: inherit owner from parent

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Date:	Tue, 15 Jan 2008 15:37:37 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	miklos@...redi.hu, serue@...ibm.com, akpm@...ux-foundation.org,
	hch@...radead.org, viro@....linux.org.uk, ebiederm@...ssion.com,
	kzak@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, containers@...ts.osdl.org,
	util-linux-ng@...r.kernel.org
Subject: Re: [patch 8/9] unprivileged mounts: propagation: inherit owner
	from parent

> > > > On mount propagation, let the owner of the clone be inherited from the
> > > > parent into which it has been propagated.  Also if the parent has the
> > > > "nosuid" flag, set this flag for the child as well.
> > > 
> > > What about nodev?
> > 
> > Hmm, I think the nosuid thing is meant to prevent suid mounts being
> > introduced into a "suidless" namespace.  This doesn't apply to dev
> > mounts, which are quite safe in a suidless environment, as long as the
> > user is not able to create devices.  But that should be taken care of
> > by capability tests.
> > 
> > I'll update the description.
> 
> Hmm,
> 
> Part of me wants to say the safest thing for now would be to refuse
> mounts propagation from non-user mounts to user mounts.
> 
> I assume you're thinking about a fully user-mounted chroot, where
> the user woudl still want to be able to stick in a cdrom and have
> it automounted under /mnt/cdrom, propagated from the root mounts ns?

Right.

> But then are there no devices which the user could create on a floppy
> while inserted into his own laptop, owned by his own uid, then insert
> into this machine, and use the device under the auto-mounted /dev/floppy
> to gain inappropriate access?

I assume, that the floppy and cdrom are already mounted with
nosuid,nodev.

The problem case is I think is if a sysadmin does some mounting in the
initial namespace, and this is propagated into the fully user-mounted
namespace (or chroot), so that a mount with suid binaries slips in.
Which is bad, because the user may be able rearange the namespace, to
trick the suid program to something it should not do.

OTOH, a mount with devices can't be abused this way, since it is not
possible to gain privileges to files/devices just by rearanging the
mounts.

Miklos
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