linux-kernel - Re: [TOMOYO #11 (linux-next) 01/11] Introduce new LSM hooks where vfsmount is available.

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Date:	Mon, 20 Oct 2008 08:27:36 -0400
From:	Shaya Potter <spotter@...columbia.edu>
To:	Kentaro Takeda <takedakn@...data.co.jp>
CC:	Stephen Smalley <sds@...ho.nsa.gov>,
	James Morris <jmorris@...ei.org>,
	Chris Wright <chrisw@...s-sol.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Toshiharu Harada <haradats@...data.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@....de>,
	Crispin Cowan <crispin@...spincowan.com>,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [TOMOYO #11 (linux-next) 01/11] Introduce new LSM hooks where
 vfsmount is available.

Kentaro Takeda wrote:
> ----- What is this patch for? -----
> 
> There are security_inode_*() LSM hooks for attribute-based MAC, but they are not
> suitable for pathname-based MAC because they don't receive "struct vfsmount"
> information.
> 
> ----- How this patch was developed? -----
> 
> Two pathname-based MACs, AppArmor and TOMOYO Linux, are trying to merge
> upstream. But because of "struct vfsmount" problem, they have been unable to
e> merge upstream.
> 
> Here are the list of approaches and the reasons of denial.

I know I'm late to the game in this, but as I recently asked about this 
and didn't get an answer, I'll re-ask my approach.

Why can't you do this

in lookup()

- resolve rules (not for single process, but for all processes) for said 
path and tag dentry (seem to already have a hook)

in permission()

- check tag based on current security context

in rename(),....

- drop dentry tag and force a lookup next time its used (invalidate dentry)

you then don't have to jump through hoops to handle things like symbolic 
links as they are handled implicitly.

the only place I can see this approach "failing" (as in different 
semantics than your approach) is

- hard links within a single namespace and bind mounts shared between 
namespaces (in that different rules would be resolved for different path 
names for the same file).

But from a security perspective, both would seem like a very bad idea in 
general that one would ant to prevent.  or to rephrase, why would you 
want to allow that?  What's the benefit in allowing that?
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