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

[Muchun Song <muchun.song@linux.dev>]

> On Aug 20, 2022, at 11:25, Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> 
> The pgdat->kswap could be accessed concurrently by kswapd_run() and
> kcompactd(), it don't be protected by any lock, which leads to the
> following null-ptr-deref,
> 
>  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
> 
> Fix it by adding READ_ONCE()|WRITE_ONCE().
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> mm/compaction.c |  4 +++-
> mm/vmscan.c     | 15 +++++++++------
> 2 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 640fa76228dd..aa1cfe47f046 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1983,7 +1983,9 @@ static inline bool is_via_compact_memory(int order)
> 
> static bool kswapd_is_running(pg_data_t *pgdat)
> {
> -	return pgdat->kswapd && task_is_running(pgdat->kswapd);
> +	struct task_struct *t = READ_ONCE(pgdat->kswapd);
> +
> +	return t && task_is_running(t);
> }
> 
> /*
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b2b1431352dc..9abba714249e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4642,16 +4642,19 @@ 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)
> +	if (READ_ONCE(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;
> +		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

So

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.

> 
> @@ -4661,11 +4664,11 @@ void kswapd_run(int nid)
>  */
> void kswapd_stop(int nid)
> {
> -	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
> +	struct task_struct *kswapd = READ_ONCE(NODE_DATA(nid)->kswapd);
> 
> 	if (kswapd) {
> 		kthread_stop(kswapd);
> -		NODE_DATA(nid)->kswapd = NULL;
> +		WRITE_ONCE(NODE_DATA(nid)->kswapd, NULL);
> 	}
> }
> 
> -- 
> 2.35.3
> 
>