[PATCH 6.1.y] mm/damon/sysfs: check contexts->nr before accessing contexts_arr[0]

[SeongJae Park <sj@kernel.org>]

From: Josh Law <objecting@objecting.org>

Multiple sysfs command paths dereference contexts_arr[0] without first
verifying that kdamond->contexts->nr == 1.  A user can set nr_contexts to
0 via sysfs while DAMON is running, causing NULL pointer dereferences.

In more detail, the issue can be triggered by privileged users like
below.

First, start DAMON and make contexts directory empty
(kdamond->contexts->nr == 0).

    # damo start
    # cd /sys/kernel/mm/damon/admin/kdamonds/0
    # echo 0 > contexts/nr_contexts

Then, each of below commands will cause the NULL pointer dereference.

    # echo update_schemes_stats > state
    # echo update_schemes_tried_regions > state
    # echo update_schemes_tried_bytes > state
    # echo update_schemes_effective_quotas > state
    # echo update_tuned_intervals > state

Guard all commands (except OFF) at the entry point of
damon_sysfs_handle_cmd().

Link: https://lkml.kernel.org/r/20260321175427.86000-3-sj@kernel.org
Fixes: 0ac32b8affb5 ("mm/damon/sysfs: support DAMOS stats")
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>	[5.18+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 1bfe9fb5ed2667fb075682408b776b5273162615)
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 a5a1e90e53e7..bdcf895a29a7 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -2576,6 +2576,9 @@ static int damon_sysfs_handle_cmd(enum damon_sysfs_cmd cmd,
 {
 	bool need_wait = true;
 
+	if (cmd != DAMON_SYSFS_CMD_OFF && kdamond->contexts->nr != 1)
+		return -EINVAL;
+
 	/* Handle commands that doesn't access DAMON context-internal data */
 	switch (cmd) {
 	case DAMON_SYSFS_CMD_ON:
-- 
2.47.3