linux-kernel - Re: Security issues with local filesystem caching

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Date:	Thu, 02 Nov 2006 14:49:27 -0500
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	David Howells <dhowells@...hat.com>
Cc:	Stephen Smalley <sds@...ho.nsa.gov>,
	Karl MacMillan <kmacmill@...hat.com>, jmorris@...ei.org,
	chrisw@...s-sol.org, selinux@...ho.nsa.gov,
	linux-kernel@...r.kernel.org, aviro@...hat.com
Subject: Re: Security issues with local filesystem caching

On Thu, 2006-11-02 at 17:16 +0000, David Howells wrote:
> Stephen Smalley <sds@...ho.nsa.gov> wrote:
> 
> > Unless there is some benefit to setting a ->fssid and checking against
> > it (e.g. safeguarding the module against unintentional internal access),
> > I think the task flag approach is preferable.
> 
> Well, I think the use of ->fssid is simpler and faster from an implementation
> standpoint (see the attached patch), and it can always be used for such as the
> in-kernel nfsd later.
> 
> The way I've done it in this patch is to have ->fssid shadow ->sid as long as
> they're the same.  But ->fssid can be set to something else and then later
> reset, at which point it becomes the same as ->sid again.  A hook is provided
> to perform both of these operations:
> 
> 	security_set_fssid(overriding_SID);  //set
> 	...
> 	security_set_fssid(SECSID_NULL);  //reset
> 
> The rest of the patch is that more or less anywhere ->sid is used to represent
> a process as an actor, this is replaced with ->fssid.  This part requires no
> conditional jumps.  It becomes a bit tricky around execve() time, but I think
> it's reasonable to ignore that as execve() is unlikely to happen in an
> overridden context; or maybe the execve() related ops should be failed if
> ->sid != ->fssid.
> 
> Do you think this is reasonable?  Or do you definitely want me to use the
> suppression flag approach instead?

Just why are you doing all this? Why do we need a back-end that requires
all this extra client-side security infrastructure in order to work? 

IOW: What is wrong with the existing CacheFS?

Trond

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