Re: [PATCH] mm: fix pgdat->kswap accessed concurrently

[Kefeng Wang]

On 2022/8/23 9:07, Kefeng Wang wrote:
> On 2022/8/21 4:59, Andrew Morton wrote:
> > On Sat, 20 Aug 2022 15:33:04 +0800 Muchun Song 
> > <muchun.song@xxxxxxxxx> wrote:
> >
> > > > +    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;
> > > > +        WRITE_ONCE(pgdat->kswapd, NULL);
> > > > +    } else {
> > > > +        WRITE_ONCE(pgdat->kswapd, t);
> > > >     }
> > > > }
> > > IIUC, the race is like the followings:
> > >
> > > CPU 0:                    CPU 1:
> > >
> > > kswapd_run()
> > >     pgdat->kswapd = kthread_run()
> > >     if (IS_ERR(pgdat->kswapd))
> > >                     kswapd_is_running
> > >                         // load pgdat->kswapd and it is NOT NULL.
> > >         pgdat->kswapd = NULL
> > >                         task_is_running(pgdat->kswapd); // NULL 
> > > pointer dereference
> > But don't we still have a bug?  Sure, kswapd_is_running() will no
> > longer deref a null pointer.  But it now runs kswapd_is_running()
> > against a task which has exited - a use-after-free?

The UAF is caused by race between kswapd_stop() and kcompactd(), right?

so  kcompactd() should be stop before kswapd_stop() to avoid the above UAF.

$ git diff
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();

> we could add get/put_task_struct() to avoid the UAF， will update， 
> thanks.

sorry, the task refcount won't fix anything.


> .