[RFC PATCH 2/2] mm/damon/core: validate damos_quota_goal->nid for node_memcg_{used,free}_bp

[SeongJae Park]

Users can set damos_quota_goal->nid with arbitrary value for
node_memcg_{used,free}_bp.  But DAMON core is using those for
NODE-DATA() without a validation of the value.  This can result in out
of bounds memory access.  The issue can actually triggered using DAMON
user-space tool (damo), like below.

    $ sudo mkdir /sys/fs/cgroup/foo
    $ sudo ./damo start --damos_action stat --damos_quota_interval 1s \
            --damos_quota_goal node_memcg_used_bp 50% -1 /foo
    $ sudo dmseg
    [...]
    [  524.181426] Unable to handle kernel paging request at virtual address 0000000000002c00

Fix this issue by adding the validation of the given node id.  If an
invalid node id is given, it returns 0% for used memory ratio, and 100%
for free memory ratio.

Fixes: b74a120bcf50 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.19.x
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
 mm/damon/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9a848d7647ef..3298ee8d8f64 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2251,6 +2251,13 @@ static unsigned long damos_get_node_memcg_used_bp(
 	unsigned long used_pages, numerator;
 	struct sysinfo i;
 
+	if (invalid_mem_node(goal->nid)) {
+		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
+			return 0;
+		else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
+			return 10000;
+	}
+
 	memcg = mem_cgroup_get_from_id(goal->memcg_id);
 	if (!memcg) {
 		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
-- 
2.47.3