Re: pidfd design

[Andy Lutomirski <luto@kernel.org>]

On Wed, Mar 20, 2019 at 11:52 AM Christian Brauner <christian@brauner.io> 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:

$ 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.

As it stands, if you create a fresh userns and mountns and try to
mount /proc, there are some really awful and hideous rules that are
checked for security reasons.  All these new APIs either need to
return something more restrictive than a proc dirfd or they need to
follow the same rules.  And I'm afraid that the latter may be a
nonstarter if you expect these APIs to be used in libraries.

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.

--Andy