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

Andy Lutomirski
2013-10-04T22:17:32Z

On Fri, Oct 4, 2013 at 12:41 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> On Fri, Oct 04, 2013 at 12:32:09PM -0700, Andy Lutomirski wrote:
>> 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.
> The current's cred each time.

Exactly. Hence the NAK.

>
> Is there some mechanism to check what you describe?
>

No. You could try to add one, but getting it to be compatible with
YAMA might be really messy.

Or you could see if destroying and recreating all the inodes on exec
or some other revoke-like approach would work.

--Andy