[PATCH 1/2] mm: fix null-ptr-deref in kswapd_is_running()

[Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>]

The kswapd_run/stop() will set pgdat->kswapd to NULL, which
could race with kswapd_is_running() in kcompactd(),

kswapd_run/stop()	kcompactd()
			  kswapd_is_running()
				if (pgdat->kswapd) // load non-NULL pgdat->kswapd
  pgdat->kswapd = NULL
				task_is_running(pgdat->kswapd) // Null pointer derefence

The KASAN report the null-ptr-deref shown below,

  vmscan: Failed to start kswapd on node 0
  ...
  BUG: KASAN: null-ptr-deref in kcompactd+0x440/0x504
  Read of size 8 at addr 0000000000000024 by task kcompactd0/37

  CPU: 0 PID: 37 Comm: kcompactd0 Kdump: loaded Tainted: G           OE     5.10.60 #1
  Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  Call trace:
   dump_backtrace+0x0/0x394
   show_stack+0x34/0x4c
   dump_stack+0x158/0x1e4
   __kasan_report+0x138/0x140
   kasan_report+0x44/0xdc
   __asan_load8+0x94/0xd0
   kcompactd+0x440/0x504
   kthread+0x1a4/0x1f0
   ret_from_fork+0x10/0x18

For race between kswapd_run() and kcompactd(), adding a temporary value
when create a kthread, and only set it to pgdat->kswapd if kthread_run()
return successful task_struct to fix the issue.

For race between kswapd_stop() and kcompactd(), let's call kcompactd_stop()
before kswapd_stop() to fix the issue.

Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
 mm/memory_hotplug.c | 2 +-
 mm/vmscan.c         | 8 +++++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index fad6d1f2262a..2fd45ccbce45 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1940,8 +1940,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 
 	node_states_clear_node(node, &arg);
 	if (arg.status_change_nid >= 0) {
-		kswapd_stop(node);
 		kcompactd_stop(node);
+		kswapd_stop(node);
 	}
 
 	writeback_set_ratelimit();
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b2b1431352dc..08c6497f76c3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4642,16 +4642,18 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 void kswapd_run(int nid)
 {
 	pg_data_t *pgdat = NODE_DATA(nid);
+	struct task_struct *t;
 
 	if (pgdat->kswapd)
 		return;
 
-	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
-	if (IS_ERR(pgdat->kswapd)) {
+	t = kthread_run(kswapd, pgdat, "kswapd%d", nid);
+	if (IS_ERR(t)) {
 		/* failure at boot is fatal */
 		BUG_ON(system_state < SYSTEM_RUNNING);
 		pr_err("Failed to start kswapd on node %d\n", nid);
-		pgdat->kswapd = NULL;
+	} else {
+		pgdat->kswapd = t;
 	}
 }
 
-- 
2.35.3