linux-kernel - Re: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory

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Date:   Thu, 29 Sep 2016 01:32:56 +0200
From:   Jann Horn <jann@...jh.net>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "security@...nel.org" <security@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Nick Kralevich <nnk@...gle.com>,
        Janis Danisevskis <jdanis@...gle.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory

On Wed, Sep 28, 2016 at 04:22:53PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 28, 2016 at 3:54 PM, Jann Horn <jann@...jh.net> wrote:
> > SELinux attempts to make it possible to whitelist trustworthy sources of
> > code that may be mapped into memory, and Android makes use of this feature.
> > To prevent an attacker from bypassing this by modifying R+X memory through
> > /proc/$pid/mem or PTRACE_POKETEXT, it is necessary to call a security hook
> > in check_vma_flags().
> 
> If selinux policy allows PTRACE_POKETEXT, is it really so bad for that
> to result in code execution?

Have a look at __ptrace_may_access():

	/* Don't let security modules deny introspection */
	if (same_thread_group(task, current))
		return 0;

This means thread A can attach to thread B and poke its memory, and SELinux
can't do anything about it.

I guess another perspective on this would be that it's a problem that
interfaces usable for poking user memory are subject to introspection rules
(as opposed to e.g. /proc/self/maps, where it is actually useful).

> > -struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
> > +struct mm_struct *proc_mem_open(struct inode *inode,
> > +                               const struct cred **object_cred,
> > +                               unsigned int mode)
> >  {
> 
> Why are you passing object_cred all over the place like this?  You
> have an inode, and an inode implies a task.

But the task's mm and objective credentials can change, and only mm_access()
holds the cred_guard_mutex during the mm lookup. Although, if the objective
credentials change because of a setuid execution, being able to poke in the
old mm would be pretty harmless...


> For that matter, would it possibly make sense to use MEMCG's mm->owner
> and get rid of object_cred entirely?

I guess it might.


> I can see this causing issues in
> strange threading cases, e.g. accessing your own /proc/$$/mem vs
> another thread in your process's.

Can you elaborate on that?

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