Commit 6557004a8b59 for kernel

commit 6557004a8b59c7701e695f02be03c7e20ed1cc15
Author: Josh Law <objecting@objecting.org>
Date:   Sat Mar 21 10:54:26 2026 -0700

    mm/damon/sysfs: check contexts->nr in repeat_call_fn

    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>

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index ddc30586c0e6..6a44a2f3d8fc 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;
 }