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

Djalal Harouni
2013-10-09T11:16:02Z

On Wed, Oct 09, 2013 at 11:54:02AM +0100, Djalal Harouni wrote:
> 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?
Or create dummy compat-quirk maps inode as Ingo put it in the other mail:
00000000-00000000 ---p 00000000 00:00 0
...

For /proc/*/maps files, to not break userspace

--
Djalal Harouni