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

In the Linux kernel, the following vulnerability has been resolved:

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().