fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks

do_fork() denies CLONE_THREAD | CLONE_PARENT if NEWUSER | NEWPID.

Then later copy_process() denies CLONE_SIGHAND if the new process will
be in a different pid namespace (task_active_pid_ns() doesn't match
current->nsproxy->pid_ns).

This looks confusing and inconsistent.  CLONE_NEWPID is very similar to
the case when ->pid_ns was already unshared, we want the same
restrictions so copy_process() should also nack CLONE_PARENT.

And it would be better to deny CLONE_NEWUSER && CLONE_SIGHAND as well
just for consistency.

Kill the "CLONE_NEWUSER | CLONE_NEWPID" check in do_fork() and change
copy_process() to do the same check along with ->pid_ns check we already
have.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Colin Walters <walters@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 15 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 68d508f2bfba..84703db06cf3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1173,13 +1173,16 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                 return ERR_PTR(-EINVAL);
 
         /*
-         * If the new process will be in a different pid namespace don't
-         * allow it to share a thread group or signal handlers with the
-         * forking task.
+         * If the new process will be in a different pid or user namespace
+         * do not allow it to share a thread group or signal handlers or
+         * parent with the forking task.
          */
-        if ((clone_flags & CLONE_SIGHAND) && (task_active_pid_ns(current) !=
-                                        current->nsproxy->pid_ns_for_children))
-                return ERR_PTR(-EINVAL);
+        if (clone_flags & (CLONE_SIGHAND | CLONE_PARENT)) {
+                if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
+                    (task_active_pid_ns(current) !=
+                                current->nsproxy->pid_ns_for_children))
+                        return ERR_PTR(-EINVAL);
+        }
 
         retval = security_task_create(clone_flags);
         if (retval)
@@ -1576,15 +1579,6 @@ long do_fork(unsigned long clone_flags,
         long nr;
 
         /*
-         * Do some preliminary argument and permissions checking before we
-         * actually start allocating stuff
-         */
-        if (clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) {
-                if (clone_flags & (CLONE_THREAD|CLONE_PARENT))
-                        return -EINVAL;
-        }
-
-        /*
          * Determine whether and which event to report to ptracer.  When
          * called from kernel_thread or CLONE_UNTRACED is explicitly
          * requested, no event is reported; otherwise, report if the event