linux-kernel - Re: [AppArmor 01/41] Pass struct vfsmount to the inode

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Message-ID: <87fy5kpye6.fsf@jbms.ath.cx>
Date:	Fri, 25 May 2007 14:10:41 -0400
From:	Jeremy Maitin-Shepard <jbms@....edu>
To:	casey@...aufler-ca.com
Cc:	Andreas Gruenbacher <agruen@...e.de>,
	James Morris <jmorris@...ei.org>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [AppArmor 01/41] Pass struct vfsmount to the inode_create LSM hook

Casey Schaufler <casey@...aufler-ca.com> writes:

> --- Jeremy Maitin-Shepard <jbms@....edu> wrote:
>> Casey Schaufler <casey@...aufler-ca.com> writes:
>> 
>> > On Fedora zcat, gzip and gunzip are all links to the same file.
>> > I can imagine (although it is a bit of a stretch) allowing a set
>> > of users access to gunzip but not gzip (or the other way around).
>> > There are probably more sophisticated programs that have different
>> > behavior based on the name they're invoked by that would provide
>> > a more compelling arguement, assuming of course that you buy into
>> > the behavior-based-on-name scheme. What I think I'm suggesting is
>> > that AppArmor might be useful in addressing the fact that a file
>> > with multiple hard links is necessarily constrained to have the
>> > same access control on each of those names. That assumes one
>> > believes that such behavior is flawwed, and I'm not going to try
>> > to argue that. The question was about an example, and there is one.
>> 
>> This doesn't work.  The behavior depends on argv[0], which is not
>> necessarily the same as the name of the file.

> Sorry, but I don't understand your objection. If AppArmor is configured
> to allow everyone access to /bin/gzip but only some people access to
> /bin/gunzip and (important detail) the single binary uses argv[0]
> as documented and (another important detail) there aren't other links
> named gunzip to the binary (ok, that's lots of if's) you should be fine.
> I suppose you could make a shell that lies to exec, but the AppArmor
> code could certainly check for that in exec by enforcing the argv[0]
> convention. It would be perfectly reasonable for a system that is so
> dependent on pathnames to require that.

Well, my point was exactly that App Armor doesn't (as far as I know) do
anything to enforce the argv[0] convention, nor would it in general
prevent a confined program from making a symlink or hard link.  Even
disregarding that, it seems very fragile in general to make an suid
program (there would be no point in confining the execution of a
non-suid program) perform essentially access control based on argv[0].

-- 
Jeremy Maitin-Shepard
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