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

Djalal Harouni
2013-10-09T10:54:10Z

On Mon, Oct 07, 2013 at 02:41:33PM -0700, Andy Lutomirski wrote:
> On Sat, Oct 5, 2013 at 6:23 AM, Djalal Harouni <tixxdz@opendz.org> wrote:
> > On Fri, Oct 04, 2013 at 03:17:08PM -0700, Andy Lutomirski wrote:
> >>
> >> Exactly. Hence the NAK.
> > But Having two LSM Hooks there is really not practical!
>
> It'd doable *if* it turns out that it's the right solution.
>
> But revoke seems much more likely to be simple, comprehensible, and
> obviously correct to me.
Yes Andy, I agree, revoke is much better!

But it will not handle or fix all the situations, as I've said what if
revoke is not invloved here? there is no an execve from the target task!

Remember:
/proc/*/{stat,maps} and perhaps others have 0444 and don't have ptrace
checks during ->open() to not break some userspace... especially
/proc/*/stat file

So you will have an fd on these privileged files!

Current will execve a privileged process, and pass ptrace_may_access()
checks during ->read()...

Here revoke is not involved at all! so it will not fix these files and
they will continue to be vulnerable.

IMO to fix them, we must have the correct ptrace_may_access() check and
this involves: current doing an execve + current's cred

BTW, Andy we already return 0 (end of file) for /proc/*/mem
->read()
->mem_read()
->mem_rw()
if (!atomic_inc_not_zero(&mm->mm_users))
return 0

So can this be considered some sort of simple revoke?

--
Djalal Harouni