Unverified Commit d9c857ae authored by Pavel Tikhomirov's avatar Pavel Tikhomirov Committed by Christian Brauner
Browse files

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: default avatarOleg Nesterov <oleg@redhat.com>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPavel 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: default avatarChristian Brauner <brauner@kernel.org>
parent 1f318b96

kernel/exit.c

+2 −1
Original line number Diff line number Diff line
@@ -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;

	}

kernel/fork.c

+4 −1
Original line number Diff line number Diff line
@@ -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;

kernel/pid.c

+2 −2
Original line number Diff line number Diff line
@@ -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 */
@@ -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))