linux-kernel - Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task

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Message-ID: <20131004191113.GA3916@dztty>
Date:	Fri, 4 Oct 2013 20:11:13 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Djalal Harouni <tixxdz@...il.com>
Subject: Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if
 file's opener may access task

On Fri, Oct 04, 2013 at 07:34:08PM +0100, Andy Lutomirski wrote:
> On Fri, Oct 4, 2013 at 7:23 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> > On Fri, Oct 04, 2013 at 04:40:01PM +0100, Andy Lutomirski wrote:
> >> On Fri, Oct 4, 2013 at 9:59 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> >> > On Thu, Oct 03, 2013 at 02:09:55PM -0700, Andy Lutomirski wrote:
[...] 
> Sorry, I described the obviously broken scenario incorrectly.  Your
> patch breaks (in the absence of things like selinux) if a exec
> something setuid root.
> 
> [...]
> 
> >
> > I did the check in the proc_same_open_cred() function:
> >  return (uid_eq(fcred->uid, cred->uid) &&
> >          gid_eq(fcred->gid, cred->gid) &&
> >          cap_issubset(cred->cap_permitted, fcred->cap_permitted));
> 
> Which has nothing to do with anything.  If that check fails, you're
> just going on to a different, WRONG check/.
> 
> >
> > Check if this is the same uid/gid and the capabilities superset!
> >
> > But in the proc_allow_access() the capabilities superset is missing.
> >
> >
> > So to fix it:
> > 1) if proc_same_open_cred() detects that cred have changed between
> > ->open() and ->read() then abort, return zero, the ->read(),write()...
> 
> IMO yuck.
> 
> >
> >
> > 2) Improve the proc_allow_access() check by:
> >    if this is the same user namespace then check uid/gid of f_cred on
> >    target cred task, and the capabilities superset:
> >    cap_issubset(tcred->cap_permitted, fcred->cap_permitted));
> >
> >    If it fails let security_capable() or file_ns_capable() do its magic.
> >
> 
> NAK.  You need to actually call the LSM.  What if the reason to fail
> the request isn't that the writer gained capabilities -- what if the
> writer's selinux label changed?
Sorry I can't follow you here! Can you be more explicit please?

For me we are already doing this during ptrace_may_access() on each
syscall, which will call LSM to inspect the privileges on each ->open(),
->write()... So LSM hooks are already called. If you want to have more
LSM hooks, then perhaps that's another problem?

-- 
Djalal Harouni
http://opendz.org
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