[PATCH] sched/fair: Fix potential memory corruption in child_cfs_rq_on_list

[Zecheng Li <zecheng@google.com>]

child_cfs_rq_on_list attempts to convert a 'prev' pointer to a cfs_rq.
This 'prev' pointer can originate from struct rq's leaf_cfs_rq_list,
making the conversion invalid and potentially leading to memory
corruption. Depending on the relative positions of leaf_cfs_rq_list and
the task group (tg) pointer within the struct, this can cause a memory
fault or access garbage data.

The issue arises in list_add_leaf_cfs_rq, where both
cfs_rq->leaf_cfs_rq_list and rq->leaf_cfs_rq_list are added to the same
leaf list. Also, rq->tmp_alone_branch can be set to rq->leaf_cfs_rq_list.

This adds a check `if (prev == &rq->leaf_cfs_rq_list)` after the main
conditional in child_cfs_rq_on_list. This ensures that the container_of
operation will convert a correct cfs_rq struct.

This check is sufficient because only cfs_rqs on the same CPU are added
to the list, so verifying the 'prev' pointer against the current rq's list
head is enough.

Fixes a potential memory corruption issue that due to current struct
layout might not be manifesting as a crash but could lead to unpredictable
behavior when the layout changes.

Signed-off-by: Zecheng Li <zecheng@google.com>
---
 kernel/sched/fair.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 857808da23d8..9dafb374d76d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4061,15 +4061,17 @@ static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq)
 {
 	struct cfs_rq *prev_cfs_rq;
 	struct list_head *prev;
+	struct rq *rq = rq_of(cfs_rq);
 
 	if (cfs_rq->on_list) {
 		prev = cfs_rq->leaf_cfs_rq_list.prev;
 	} else {
-		struct rq *rq = rq_of(cfs_rq);
-
 		prev = rq->tmp_alone_branch;
 	}
 
+	if (prev == &rq->leaf_cfs_rq_list)
+		return false;
+
 	prev_cfs_rq = container_of(prev, struct cfs_rq, leaf_cfs_rq_list);
 
 	return (prev_cfs_rq->tg->parent == cfs_rq->tg);

base-commit: 7ab02bd36eb444654183ad6c5b15211ddfa32a8f
-- 
2.48.1