[PATCH] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

[Jeongjun Park <aha310510@gmail.com>]

Read/Write to pcpu_nr_populated should be performed while protected
by pcpu_lock. However, pcpu_nr_pages() reads pcpu_nr_populated without any
protection, which causes a data race between read/write.

Therefore, when reading pcpu_nr_populated in pcpu_nr_pages(), it should be
modified to be protected by pcpu_lock.

Reported-by: syzbot+e5bd32b79413e86f389e@syzkaller.appspotmail.com
Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
 mm/percpu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index b35494c8ede2..0f98b857fb36 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -3355,7 +3355,13 @@ void __init setup_per_cpu_areas(void)
  */
 unsigned long pcpu_nr_pages(void)
 {
-	return pcpu_nr_populated * pcpu_nr_units;
+	unsigned long flags, ret;
+
+	spin_lock_irqsave(&pcpu_lock, flags);
+	ret = pcpu_nr_populated * pcpu_nr_units;
+	spin_unlock_irqrestore(&pcpu_lock, flags);
+
+	return ret;
 }
 
 /*
--