This patch is the first step in enabling checkpoint/restore of processes
with seccomp enabled.

One of the things CRIU does while dumping tasks is inject code into them
via ptrace to collect information that is only available to the process
itself. However, if we are in a seccomp mode where these processes are
prohibited from making these syscalls, then what CRIU does kills the task.

This patch adds a new ptrace option, PTRACE_O_SUSPEND_SECCOMP, that enables
a task from the init user namespace which has CAP_SYS_ADMIN and no seccomp
filters to disable (and re-enable) seccomp filters for another task so that
they can be successfully dumped (and restored). We restrict the set of
processes that can disable seccomp through ptrace because although today
ptrace can be used to bypass seccomp, there is some discussion of closing
this loophole in the future and we would like this patch to not depend on
that behavior and be future proofed for when it is removed.

Note that seccomp can be suspended before any filters are actually
installed; this behavior is useful on criu restore, so that we can suspend
seccomp, restore the filters, unmap our restore code from the restored
process' address space, and then resume the task by detaching and have the
filters resumed as well.

v2 changes:

* require that the tracer have no seccomp filters installed
* drop TIF_NOTSC manipulation from the patch
* change from ptrace command to a ptrace option and use this ptrace option
  as the flag to check. This means that as soon as the tracer
  detaches/dies, seccomp is re-enabled and as a corrollary that one can not
  disable seccomp across PTRACE_ATTACHs.

v3 changes:

* get rid of various #ifdefs everywhere
* report more sensible errors when PTRACE_O_SUSPEND_SECCOMP is incorrectly
  used

v4 changes:

* get rid of may_suspend_seccomp() in favor of a capable() check in ptrace
  directly

v5 changes:

* check that seccomp is not enabled (or suspended) on the tracer

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Will Drewry <wad@chromium.org>
CC: Roland McGrath <roland@hack.frob.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
[kees: access seccomp.mode through seccomp_mode() instead]
Signed-off-by: Kees Cook <keescook@chromium.org>