[PATCH 6.18 245/309] mm/damon/sysfs: check contexts->nr in repeat_call_fn

Greg Kroah-Hartman
2026-03-31T17:07:11Z

From: Josh Law <objecting@objecting.org>

commit 6557004a8b59c7701e695f02be03c7e20ed1cc15 upstream.

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

Link: https://patch.msgid.link/20260320163559.178101-3-objecting@objecting.org
Link: https://lkml.kernel.org/r/20260321175427.86000-4-sj@kernel.org
Link: https://github.com/damonitor/damo [1]
Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> [6.17+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/damon/sysfs.c | 3 +++
1 file changed, 3 insertions(+)

--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1570,9 +1570,12 @@ static int damon_sysfs_repeat_call_fn(vo

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;
}