ipmi: fix sleep-in-atomic in free_user at cleanup SRCU user->release_barrier

free_user() could be called in atomic context.

This patch pushed the free operation off into a workqueue.

Example:

 BUG: sleeping function called from invalid context at kernel/workqueue.c:2856
 in_atomic(): 1, irqs_disabled(): 0, pid: 177, name: ksoftirqd/27
 CPU: 27 PID: 177 Comm: ksoftirqd/27 Not tainted 4.19.25-3 #1
 Hardware name: AIC 1S-HV26-08/MB-DPSB04-06, BIOS IVYBV060 10/21/2015
 Call Trace:
  dump_stack+0x5c/0x7b
  ___might_sleep+0xec/0x110
  __flush_work+0x48/0x1f0
  ? try_to_del_timer_sync+0x4d/0x80
  _cleanup_srcu_struct+0x104/0x140
  free_user+0x18/0x30 [ipmi_msghandler]
  ipmi_free_recv_msg+0x3a/0x50 [ipmi_msghandler]
  deliver_response+0xbd/0xd0 [ipmi_msghandler]
  deliver_local_response+0xe/0x30 [ipmi_msghandler]
  handle_one_recv_msg+0x163/0xc80 [ipmi_msghandler]
  ? dequeue_entity+0xa0/0x960
  handle_new_recv_msgs+0x15c/0x1f0 [ipmi_msghandler]
  tasklet_action_common.isra.22+0x103/0x120
  __do_softirq+0xf8/0x2d7
  run_ksoftirqd+0x26/0x50
  smpboot_thread_fn+0x11d/0x1e0
  kthread+0x103/0x140
  ? sort_range+0x20/0x20
  ? kthread_destroy_worker+0x40/0x40
  ret_from_fork+0x1f/0x40

Fixes: 77f82696 ("ipmi: fix use-after-free of user->release_barrier.rda")
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: stable@vger.kernel.org # 5.0
Cc: Yang Yingliang <yangyingliang@huawei.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -214,6 +214,9 @@ struct ipmi_user {
 
 	/* Does this interface receive IPMI events? */
 	bool gets_events;
+
+	/* Free must run in process context for RCU cleanup. */
+	struct work_struct remove_work;
 };
 
 static struct ipmi_user *acquire_ipmi_user(struct ipmi_user *user, int *index)
@@ -1157,6 +1160,15 @@ static int intf_err_seq(struct ipmi_smi *intf,
 	return rv;
 }
 
+static void free_user_work(struct work_struct *work)
+{
+	struct ipmi_user *user = container_of(work, struct ipmi_user,
+					      remove_work);
+
+	cleanup_srcu_struct(&user->release_barrier);
+	kfree(user);
+}
+
 int ipmi_create_user(unsigned int          if_num,
 		     const struct ipmi_user_hndl *handler,
 		     void                  *handler_data,
@@ -1200,6 +1212,8 @@ int ipmi_create_user(unsigned int          if_num,
 	goto out_kfree;
 
  found:
+	INIT_WORK(&new_user->remove_work, free_user_work);
+
 	rv = init_srcu_struct(&new_user->release_barrier);
 	if (rv)
 		goto out_kfree;
@@ -1260,8 +1274,9 @@ EXPORT_SYMBOL(ipmi_get_smi_info);
 static void free_user(struct kref *ref)
 {
 	struct ipmi_user *user = container_of(ref, struct ipmi_user, refcount);
-	cleanup_srcu_struct(&user->release_barrier);
-	kfree(user);
+
+	/* SRCU cleanup must happen in task context. */
+	schedule_work(&user->remove_work);
 }
 
 static void _ipmi_destroy_user(struct ipmi_user *user)