pid_namespace: avoid optimization of accesses to ->child_reaper

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@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Link: https://patch.msgid.link/20260318122157.280595-2-ptikhomirov@virtuozzo.com
v3: Split from main commit. Add ASSERT_EXCLUSIVE_WRITER.
v5: Add one more READ_ONCE for access without lock in free_pid().
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index ede3117fa7d4..31b714c3a791 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;
 	}