[PATCH] pid: Replace struct pid 1-element array with flex-array

[Kees Cook]

For pid namespaces, struct pid uses a dynamically sized array member,
"numbers". This was implemented using the ancient 1-element fake flexible
array, which has been deprecated for decades. Replace it with a C99
flexible array, refactor the array size calculations to use struct_size(),
and address elements via indexes. Note that the static initializer (which
defines a single element) works as-is, and requires no special handling.

Without this, CONFIG_UBSAN_BOUNDS (and potentially CONFIG_FORTIFY_SOURCE)
will trigger bounds checks:
https://lore.kernel.org/lkml/20230517-bushaltestelle-super-e223978c1ba6@brauner

Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
Cc: Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Jeff Xu <jeffxu@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Reported-by: syzbot+ac3b41786a2d0565b6d5@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/pid.h    |  2 +-
 kernel/pid.c           | 13 ++++++++-----
 kernel/pid_namespace.c |  2 +-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index b75de288a8c2..653a527574c4 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -67,7 +67,7 @@ struct pid
 	/* wait queue for pidfd notifications */
 	wait_queue_head_t wait_pidfd;
 	struct rcu_head rcu;
-	struct upid numbers[1];
+	struct upid numbers[];
 };
 
 extern struct pid init_struct_pid;
diff --git a/kernel/pid.c b/kernel/pid.c
index f93954a0384d..f5008146e2e4 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -131,7 +131,7 @@ void free_pid(struct pid *pid)
 
 	spin_lock_irqsave(&pidmap_lock, flags);
 	for (i = 0; i <= pid->level; i++) {
-		struct upid *upid = pid->numbers + i;
+		struct upid *upid = &pid->numbers[i];
 		struct pid_namespace *ns = upid->ns;
 		switch (--ns->pid_allocated) {
 		case 2:
@@ -265,7 +265,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	init_waitqueue_head(&pid->wait_pidfd);
 	INIT_HLIST_HEAD(&pid->inodes);
 
-	upid = pid->numbers + ns->level;
+	upid = &pid->numbers[ns->level];
 	spin_lock_irq(&pidmap_lock);
 	if (!(ns->pid_allocated & PIDNS_ADDING))
 		goto out_unlock;
@@ -285,7 +285,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 out_free:
 	spin_lock_irq(&pidmap_lock);
 	while (++i <= ns->level) {
-		upid = pid->numbers + i;
+		upid = &pid->numbers[i];
 		idr_remove(&upid->ns->idr, upid->nr);
 	}
 
@@ -656,8 +656,11 @@ void __init pid_idr_init(void)
 
 	idr_init(&init_pid_ns.idr);
 
-	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
-			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
+	init_pid_ns.pid_cachep = kmem_cache_create("pid",
+			struct_size((struct pid *)0, numbers, 1),
+			__alignof__(struct pid),
+			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
+			NULL);
 }
 
 static struct file *__pidfd_fget(struct task_struct *task, int fd)
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index b43eee07b00c..ebb20091a9da 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -48,7 +48,7 @@ static struct kmem_cache *create_pid_cachep(unsigned int level)
 		return kc;
 
 	snprintf(name, sizeof(name), "pid_%u", level + 1);
-	len = sizeof(struct pid) + level * sizeof(struct upid);
+	len = struct_size((struct pid *)0, numbers, level + 1);
 	mutex_lock(&pid_caches_mutex);
 	/* Name collision forces to do allocation under mutex. */
 	if (!*pkc)
-- 
2.34.1