[PATCH] block: Check blkg_to_lat return value to avoid NULL dereference

[Wentao Liang <vulab@iscas.ac.cn>]

The function blkg_to_lat() may return NULL if the blkg is not associated
with an iolatency group. In iolatency_set_min_lat_nsec() and
iolatency_pd_init(), the return values are not checked, leading to
potential NULL pointer dereferences.

This patch adds checks for the return values of blkg_to_lat and let it
returns early if it is NULL, preventing the NULL pointer dereference.

Fixes: d70675121546 ("block: introduce blk-iolatency io controller")
Cc: stable@vger.kernel.org # 4.19+
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
---
 block/blk-iolatency.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index ebb522788d97..398f0a1747c4 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -787,6 +787,8 @@ static int blk_iolatency_init(struct gendisk *disk)
 static void iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
 {
 	struct iolatency_grp *iolat = blkg_to_lat(blkg);
+	if (!iolat)
+		return;
 	struct blk_iolatency *blkiolat = iolat->blkiolat;
 	u64 oldval = iolat->min_lat_nsec;
 
@@ -1013,6 +1015,8 @@ static void iolatency_pd_init(struct blkg_policy_data *pd)
 	 */
 	if (blkg->parent && blkg_to_pd(blkg->parent, &blkcg_policy_iolatency)) {
 		struct iolatency_grp *parent = blkg_to_lat(blkg->parent);
+		if (!parent)
+			return;
 		atomic_set(&iolat->scale_cookie,
 			   atomic_read(&parent->child_lat.scale_cookie));
 	} else {
-- 
2.42.0.windows.2