Merge tag 'kernel-7.1-rc1.misc' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull pid_namespace updates from Christian Brauner:

 - pid_namespace: make init creation more flexible

   Annotate ->child_reaper accesses with {READ,WRITE}_ONCE() to protect
   the unlocked readers from cpu/compiler reordering, and enforce that
   pid 1 in a pid namespace is always the first allocated pid (the
   set_tid path already required this).

   On top of that, allow opening pid_for_children before the pid
   namespace init has been created. This lets one process create the pid
   namespace and a different process create the init via setns(), which
   makes clone3(set_tid) usable in all cases evenly and is particularly
   useful to CRIU when restoring nested containers.

   A new selftest covers both the basic create-pidns-then-init flow and
   the cross-process variant, and a MAINTAINERS entry for the pid
   namespace code is added.

 - unrelated signal cleanup: update outdated comment for the removed
   freezable_schedule()

* tag 'kernel-7.1-rc1.misc' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  signal: update outdated comment for removed freezable_schedule()
  MAINTAINERS: add a pid namespace entry
  selftests: Add tests for creating pidns init via setns
  pid_namespace: allow opening pid_for_children before init was created
  pid: check init is created first after idr alloc
  pid_namespace: avoid optimization of accesses to ->child_reaper

1 files changed, 11 insertions, 8 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index 3b96571d0fe6..677c84e319dd 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -128,7 +128,7 @@ void free_pid(struct pid *pid)
 			 * is the reaper wake up the reaper.  The reaper
 			 * may be sleeping in zap_pid_ns_processes().
 			 */
-			wake_up_process(ns->child_reaper);
+			wake_up_process(READ_ONCE(ns->child_reaper));
 			break;
 		case PIDNS_ADDING:
 			/* Handle a fork failure of the first process */
@@ -215,12 +215,6 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 			retval = -EINVAL;
 			if (tid < 1 || tid >= pid_max[ns->level - i])
 				goto out_abort;
-			/*
-			 * Also fail if a PID != 1 is requested and
-			 * no PID 1 exists.
-			 */
-			if (tid != 1 && !tmp->child_reaper)
-				goto out_abort;
 			retval = -EPERM;
 			if (!checkpoint_restore_ns_capable(tmp->user_ns))
 				goto out_abort;
@@ -296,9 +290,18 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 
 		pid->numbers[i].nr = nr;
 		pid->numbers[i].ns = tmp;
-		tmp = tmp->parent;
 		i--;
 		retried_preload = false;
+
+		/*
+		 * PID 1 (init) must be created first.
+		 */
+		if (!READ_ONCE(tmp->child_reaper) && nr != 1) {
+			retval = -EINVAL;
+			goto out_free;
+		}
+
+		tmp = tmp->parent;
 	}
 
 	/*