[PATCH v3 1/4] pid_namespace: avoid optimization of accesses to ->child_reaper

[Pavel Tikhomirov]

To avoid potential problems related to cpu/compiler optimizations around
->child_reaper, let's use WRITE_ONCE (additional to task_list lock)
everywhere we write it and use READ_ONCE where we read it without
explicit lock. Note: It also pairs with existing READ_ONCE with no lock
in nsfs_fh_to_dentry().

Also let's add ASSERT_EXCLUSIVE_WRITER before write to identify to KCSAN
that we don't expect any concurrent ->child_reaper modifications, and
those must be detected.

Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
--
v3: Split from main commit. Add ASSERT_EXCLUSIVE_WRITER.
---
 kernel/exit.c | 3 ++-
 kernel/fork.c | 5 ++++-
 kernel/pid.c  | 2 +-
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 8a87021211ae..8e5e523dcc79 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -608,7 +608,8 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
 
 	reaper = find_alive_thread(father);
 	if (reaper) {
-		pid_ns->child_reaper = reaper;
+		ASSERT_EXCLUSIVE_WRITER(pid_ns->child_reaper);
+		WRITE_ONCE(pid_ns->child_reaper, reaper);
 		return reaper;
 	}
 
diff --git a/kernel/fork.c b/kernel/fork.c
index e832da9d15a4..9ce2d12ec701 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2423,7 +2423,10 @@ __latent_entropy struct task_struct *copy_process(
 			init_task_pid(p, PIDTYPE_SID, task_session(current));
 
 			if (is_child_reaper(pid)) {
-				ns_of_pid(pid)->child_reaper = p;
+				struct pid_namespace *ns = ns_of_pid(pid);
+
+				ASSERT_EXCLUSIVE_WRITER(ns->child_reaper);
+				WRITE_ONCE(ns->child_reaper, p);
 				p->signal->flags |= SIGNAL_UNKILLABLE;
 			}
 			p->signal->shared_pending.signal = delayed.signal;
diff --git a/kernel/pid.c b/kernel/pid.c
index 3b96571d0fe6..76c2744493e2 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -219,7 +219,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 			 * Also fail if a PID != 1 is requested and
 			 * no PID 1 exists.
 			 */
-			if (tid != 1 && !tmp->child_reaper)
+			if (tid != 1 && !READ_ONCE(tmp->child_reaper))
 				goto out_abort;
 			retval = -EPERM;
 			if (!checkpoint_restore_ns_capable(tmp->user_ns))
-- 
2.53.0