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

Djalal Harouni
2013-10-04T19:11:28Z

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?

--
Djalal Harouni