On Tue, Apr 25, 2017 at 5:23 AM, Djalal Harouni [off-list ref] wrote:

This patch allows to have multiple private procfs instances inside the
same pid namespace. Lot of other areas in the kernel and filesystems
have been updated to be able to support private instances, devpts is one
major example. The aim here is lightweight sandboxes, and to allow that we
have to modernize procfs internals.

1) The main aim of this work is to have on embedded systems one
supervisor for apps. Right now we have some lightweight sandbox support,
however if we create pid namespacess we have to manages all the
processes inside too, where our goal is to be able to run a bunch of
apps each one inside its own mount namespace without being able to
notice each other. We only want to use mount namespaces, and we want
procfs to behave more like a real mount point.

2) Linux Security Modules have multiple ptrace paths inside some
subsystems, however inside procfs, the implementation does not guarantee
that the ptrace() check which triggers the security_ptrace_check() hook
will always run. We have the 'hidepid' mount option that can be used to
force the ptrace_may_access() check inside has_pid_permissions() to run.
The problem is that 'hidepid' is per pid namespace and not attached to
the mount point, any remount or modification of 'hidepid' will propagate
to all other procfs mounts.

This also does not allow to support Yama LSM easily in desktop and user
sessions. Yama ptrace scope which restricts ptrace and some other
syscalls to be allowed only on inferiors, can be updated to have a
per-task context, where the context will be inherited during fork(),
clone() and preserved across execve(). If we support multiple private
procfs instances, then we may force the ptrace_may_access() on
/proc/<pids>/ to always run inside that new procfs instances. This will
allow to specifiy on user sessions if we should populate procfs with
pids that the user can ptrace or not.

By using Yama ptrace scope, some restricted users will only be able to see
inferiors inside /proc, they won't even be able to see their other
processes. Some software like Chromium, Firefox's crash handler, Wine
and others are already using Yama to restrict which processes can be
ptracable. With this change this will give the possibility to restrict
/proc/<pids>/ but more importantly this will give desktop users a
generic and usuable way to specifiy which users should see all processes
and which users can not.

Side notes:
* This covers the lack of seccomp where it is not able to parse
arguments, it is easy to install a seccomp filter on direct syscalls
that operate on pids, however /proc/<pid>/ is a Linux ABI using
filesystem syscalls. With this change LSMs should be able to analyze
open/read/write/close...

3) This will modernize procfs and align it with all other filesystems
and subsystems that have been updated recently to be able to work in a
flexible way. This is the same as devpts where each mount now is a distinct
filesystem such that ptys and their indicies allocated in one mount are
independent from ptys and their indicies in all other mounts.

We have to align procfs and modernize it to have a per mount context
where at least the mount option do not propagate to all other mounts,
then maybe we can continue to implement new features. One example is to
require CAP_SYS_ADMIN in the init user namespace on some /proc/* which are
not pids and which are are not virtualized by design, or CAP_NET_ADMIN
inside userns on the net bits that are virtualized, etc.
These mount options won't propagate to previous mounts, and the system
will continue to be usable.

Ths patch introduces the new 'limit_pids' mount option as it was also
suggesed by Andy Lutomirski [1]. When this option is passed we
automatically create a private procfs instance. This is not the default
behaviour since we do not want to break userspace and we do not want to
provide different devices IDs by default, please see [1] for why.

I think that calling the option to make a separate instance
"limit_pids" is extremely counterintuitive.

My strong preference would be to make proc *always* make a separate
instance (unless it's a bind mount) and to make it work.  If that
means fudging stat() output, so be it.

Failing that, let's come up with some coherent way to make this work.
"new_instance" or similar would do.  Then make limit_pid cause an
error unless new_instance is also set.

--Andy