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

Andy Lutomirski
2013-10-04T19:32:32Z

On Fri, Oct 4, 2013 at 12:27 PM, Djalal Harouni <tixxdz@opendz.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@opendz.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@opendz.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@opendz.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.

--Andy

>
>> --Andy
>
> --
> Djalal Harouni
> http://opendz.org

--
Andy Lutomirski
AMA Capital Management, LLC