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

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/[email protected]
Fixes: 0ac32b8 ("mm/damon/sysfs: support DAMOS stats")
Signed-off-by: Josh Law <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
Cc: <[email protected]>	[5.18+]
Signed-off-by: Andrew Morton <[email protected]>

Author: techyguyperplexable

mm/damon/sysfs.c

@@ -1749,6 +1749,9 @@ static int damon_sysfs_update_schemes_tried_regions(
 static int damon_sysfs_handle_cmd(enum damon_sysfs_cmd cmd,
 		struct damon_sysfs_kdamond *kdamond)
 {
+	if (cmd != DAMON_SYSFS_CMD_OFF && kdamond->contexts->nr != 1)
+		return -EINVAL;
+
 	switch (cmd) {
 	case DAMON_SYSFS_CMD_ON:
 		return damon_sysfs_turn_damon_on(kdamond);