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

Jann Horn
2016-09-28T23:44:15Z

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On Thu, Sep 29, 2016 at 01:32:56AM +0200, Jann Horn wrote:
> 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@thejh.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).

Ugh, I'm talking nonsense, ptrace() doesn't work on threads. (/proc/$pid/mem
works though). And then, ptrace-ish APIs aside, there are those weird
devices that do DMA with force=1.

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