[PATCH RESEND v11 2/8] proc: allow to mount many instances of proc in one pid namespace

[Alexey Gladkov]

On Fri, Apr 17, 2020 at 01:55:05PM -0500, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey at gmail.com> writes:
> 
> > This patch allows to have multiple procfs instances inside the
> > same pid namespace. 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...
> >
> > In the new patchset version I removed the 'newinstance' option
> > as suggested by Eric W. Biederman.
> 
> Some very small requests.
> 
> 1) Can you please not place fs_info in fs_context, and instead allocate
>    fs_info in fill_super?  Unless I have misread introduced a resource
>    leak if proc is not mounted or if proc is simply reconfigured.

Hm ... it seems you're right.

> 2) Can you please move hide_pid and pid_gid into fs_info in this patch?
>    As was shown by my recent bug fix 

OK. I’ll do it in the next version.

> 3) Can you please rebase on on v5.7-rc1 or v5.7-rc2 and repost these
>    patches please?  I thought I could do it safely but between my bug
>    fixes, and Alexey Dobriyan's parallel changes to proc these patches
>    do not apply cleanly.
> 
>    Plus there is a resource leak in this patch.

On my way.

> >  struct proc_fs_context {
> > -	struct pid_namespace	*pid_ns;
> > +	struct proc_fs_info	*fs_info;
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Please don't do this. As best as I can tell that introduces a memory
> leak of proc is not mounted.  Please allocate fs_info in 

OK.

-- 
Rgrds, legion