CVE-2025-21919

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

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.