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: <20131004194142.GA4524@dztty>
Date:	Fri, 4 Oct 2013 20:41:42 +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 12:32:09PM -0700, Andy Lutomirski wrote:
> On Fri, Oct 4, 2013 at 12:27 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> > On Fri, Oct 04, 2013 at 12:16:26PM -0700, Andy Lutomirski wrote:
> >> On Fri, Oct 4, 2013 at 12:11 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> >> > 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?
> >>
> >> Can you show me where, in your code, LSMs are asked whether the
> >> process calling read() is permitted to ptrace the process that the
> >> proc file points at?
> > Yes.
> > [PATCH v2 9/9] procfs: improve permission checks on /proc/*/syscall
> >
> > ->read()
> >   ->syscall_read()
> >     ->lock_trace()
> >       ->ptrace_may_access()
> >         ->__ptrace_may_access()
> >           ->security_ptrace_access_check()
> >             ->yama_ptrace_access_check()
> >             ->security_ops->ptrace_access_check()
> >
> >
> > And also for patch:
> > [PATCH v2 8/9] procfs: improve permission checks on /proc/*/stack
> >
> > And during ->open() and ->read()
> >
> >
> > So sorry Andy, I don't follow what you are describing.
> 
> And what parameters are you passing to security_ptrace_access_check?
> It's supposed to be f_cred, right?  Because you want to make sure
> that, if the opener had some low-privilege label, the target has
> execed and gotten a more secure label, and the reader has a
> high-privilege label, that the opener's label is checked against the
> target's new label.
The current's cred each time.

Is there some mechanism to check what you describe?


> --Andy
> 
> >
> >> --Andy
> >
> > --
> > Djalal Harouni
> > http://opendz.org
> 
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC

-- 
Djalal Harouni
http://opendz.org
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