devlink: keep the instance mutex alive until references are gone

The reference needs to keep the instance memory around, but also
the instance lock must remain valid. Users will take the lock,
check registration status and release the lock. mutex_destroy()
etc. belong in the same place as the freeing of the memory.

Unfortunately lockdep_unregister_key() sleeps so we need
to switch the an rcu_work.

Note that the problem is a bit hard to repro, because
devlink_pernet_pre_exit() iterates over registered instances.
AFAIU the instances must get devlink_free()d concurrently with
the namespace getting deleted for the problem to occur.

Reported-by: syzbot+d94d214ea473e218fc89@syzkaller.appspotmail.com
Reported-by: syzbot+9f0dd863b87113935acf@syzkaller.appspotmail.com
Fixes: 9053637e ("devlink: remove the registration guarantee of references")
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230111042908.988199-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/devlink/core.c

@@ -83,10 +83,21 @@ struct devlink *__must_check devlink_try_get(struct devlink *devlink)
 	return NULL;
 }
 
+static void devlink_release(struct work_struct *work)
+{
+	struct devlink *devlink;
+
+	devlink = container_of(to_rcu_work(work), struct devlink, rwork);
+
+	mutex_destroy(&devlink->lock);
+	lockdep_unregister_key(&devlink->lock_key);
+	kfree(devlink);
+}
+
 void devlink_put(struct devlink *devlink)
 {
 	if (refcount_dec_and_test(&devlink->refcount))
-		kfree_rcu(devlink, rcu);
+		queue_rcu_work(system_wq, &devlink->rwork);
 }
 
 struct devlink *devlinks_xa_find_get(struct net *net, unsigned long *indexp)
@@ -231,6 +242,7 @@ struct devlink *devlink_alloc_ns(const struct devlink_ops *ops,
 	INIT_LIST_HEAD(&devlink->trap_list);
 	INIT_LIST_HEAD(&devlink->trap_group_list);
 	INIT_LIST_HEAD(&devlink->trap_policer_list);
+	INIT_RCU_WORK(&devlink->rwork, devlink_release);
 	lockdep_register_key(&devlink->lock_key);
 	mutex_init(&devlink->lock);
 	lockdep_set_class(&devlink->lock, &devlink->lock_key);
@@ -259,8 +271,6 @@ void devlink_free(struct devlink *devlink)
 
 	mutex_destroy(&devlink->linecards_lock);
 	mutex_destroy(&devlink->reporters_lock);
-	mutex_destroy(&devlink->lock);
-	lockdep_unregister_key(&devlink->lock_key);
 	WARN_ON(!list_empty(&devlink->trap_policer_list));
 	WARN_ON(!list_empty(&devlink->trap_group_list));
 	WARN_ON(!list_empty(&devlink->trap_list));

net/devlink/devl_internal.h

@@ -7,6 +7,7 @@
 #include <linux/netdevice.h>
 #include <linux/notifier.h>
 #include <linux/types.h>
+#include <linux/workqueue.h>
 #include <linux/xarray.h>
 #include <net/devlink.h>
 #include <net/net_namespace.h>
@@ -51,7 +52,7 @@ struct devlink {
 	struct lock_class_key lock_key;
 	u8 reload_failed:1;
 	refcount_t refcount;
-	struct rcu_head rcu;
+	struct rcu_work rwork;
 	struct notifier_block netdevice_nb;
 	char priv[] __aligned(NETDEV_ALIGN);
 };