Re: pidfd design

[Daniel Colascione]

On Wed, Mar 20, 2019 at 12:14 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On Wed, Mar 20, 2019 at 11:58:57AM -0700, Andy Lutomirski wrote:
> > On Wed, Mar 20, 2019 at 11:52 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > >
> > > You're misunderstanding. Again, I said in my previous mails it should
> > > accept pidfds optionally as arguments, yes. But I don't want it to
> > > return the status fds that you previously wanted pidfd_wait() to return.
> > > I really want to see Joel's pidfd_wait() patchset and have more people
> > > review the actual code.
> >
> > Just to make sure that no one is forgetting a material security consideration:
>
> Andy, thanks for commenting!
>
> >
> > $ ls /proc/self
> > attr             exe        mountinfo      projid_map    status
> > autogroup        fd         mounts         root          syscall
> > auxv             fdinfo     mountstats     sched         task
> > cgroup           gid_map    net            schedstat     timers
> > clear_refs       io         ns             sessionid     timerslack_ns
> > cmdline          latency    numa_maps      setgroups     uid_map
> > comm             limits     oom_adj        smaps         wchan
> > coredump_filter  loginuid   oom_score      smaps_rollup
> > cpuset           map_files  oom_score_adj  stack
> > cwd              maps       pagemap        stat
> > environ          mem        personality    statm
> >
> > A bunch of this stuff makes sense to make accessible through a syscall
> > interface that we expect to be used even in sandboxes.  But a bunch of
> > it does not.  For example, *_map, mounts, mountstats, and net are all
> > namespace-wide things that certain policies expect to be unavailable.
> > stack, for example, is a potential attack surface.  Etc.

If you can access these files sources via open(2) on /proc/<pid>, you
should be able to access them via a pidfd. If you can't, you
shouldn't. Which /proc? The one you'd get by mounting procfs. I don't
see how pidfd makes any material changes to anyone's security. As far
as I'm concerned, if a sandbox can't mount /proc at all, it's just a
broken and unsupported configuration.

An actual threat model and real thought paid to access capabilities
would help. Almost everything around the interaction of Linux kernel
namespaces and security feels like a jumble of ad-hoc patches added as
afterthoughts in response to random objections.

>> All these new APIs either need to
> > return something more restrictive than a proc dirfd or they need to
> > follow the same rules.

What's wrong with the latter?

> > And I'm afraid that the latter may be a
> > nonstarter if you expect these APIs to be used in libraries.

What's special about libraries? How is a library any worse-off using
openat(2) on a pidfd than it would be just opening the file called
"/proc/$apid"?

> > Yes, this is unfortunate, but it is indeed the current situation.  I
> > suppose that we could return magic restricted dirfds, or we could
> > return things that aren't dirfds and all and have some API that gives
> > you the dirfd associated with a procfd but only if you can see
> > /proc/PID.
>
> What would be your opinion to having a
> /proc/<pid>/handle
> file instead of having a dirfd. Essentially, what I initially proposed
> at LPC. The change on what we currently have in master would be:
> https://gist.github.com/brauner/59eec91550c5624c9999eaebd95a70df

And how do you propose, given one of these handle objects, getting a
process's current priority, or its current oom score, or its list of
memory maps? As I mentioned in my original email, and which nobody has
addressed, if you don't use a dirfd as your process handle or you
don't provide an easy way to get one of these proc directory FDs, you
need to duplicate a lot of metadata access interfaces.