Commit 3b4035dd authored Mar 04, 2025 by Zecheng Li Committed by Peter Zijlstra Mar 05, 2025

sched/fair: Fix potential memory corruption in child_cfs_rq_on_list

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.

Fixes: fdaba61e ("sched/fair: Ensure that the CFS parent is added after unthrottling")
Signed-off-by: Zecheng Li <zecheng@google.com>
Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20250304214031.2882646-1-zecheng@google.com

parent 7eb17214

kernel/sched/fair.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -4045,15 +4045,17 @@ 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);