[PATCH v3 3/3] mm/damon/sysfs: check contexts->nr in repeat_call_fn

[SeongJae Park <sj@kernel.org>]

From: Josh Law <objecting@objecting.org>

damon_sysfs_repeat_call_fn() calls damon_sysfs_upd_tuned_intervals(),
damon_sysfs_upd_schemes_stats(), and
damon_sysfs_upd_schemes_effective_quotas() without checking
contexts->nr.  If nr_contexts is set to 0 via sysfs while DAMON is
running, these functions dereference contexts_arr[0] and cause a NULL
pointer dereference.  Add the missing check.

For example, the issue can be reproduced using DAMON sysfs interface and
DAMON user-space tool (damo) [1] like below.

    $ sudo damo start --refresh_interval 1s
    $ echo 0 | sudo tee \
            /sys/kernel/mm/damon/admin/kdamonds/0/contexts/nr_contexts

[1] https://github.com/damonitor/damo

Link: https://patch.msgid.link/20260320163559.178101-3-objecting@objecting.org
Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index ddc30586c0e61..6a44a2f3d8fc9 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;
 }
-- 
2.47.3