[PATCH 4/4] mm/damon/sysfs: check contexts->nr in repeat_call_fn

[Josh Law <objecting@objecting.org>]

damon_sysfs_repeat_call_fn() accesses contexts_arr[0] in
upd_tuned_intervals, upd_schemes_stats, and upd_schemes_effective_quotas
without checking nr_contexts. A user can set nr_contexts to 0 via sysfs
while DAMON is running, causing a NULL pointer dereference in the
repeat callback. Add a guard under the lock.

Signed-off-by: Josh Law <objecting@objecting.org>
---
 mm/damon/sysfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index ddcdc4e35b27..d982f2dc7a2b 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1620,9 +1620,12 @@ static int damon_sysfs_repeat_call_fn(void *data)
 
 	if (!mutex_trylock(&damon_sysfs_lock))
 		return 0;
+	if (sysfs_kdamond->contexts->nr != 1)
+		goto out;
 	damon_sysfs_upd_tuned_intervals(sysfs_kdamond);
 	damon_sysfs_upd_schemes_stats(sysfs_kdamond);
 	damon_sysfs_upd_schemes_effective_quotas(sysfs_kdamond);
+out:
 	mutex_unlock(&damon_sysfs_lock);
 	return 0;
 }
-- 
2.34.1