CVE-2025-21919sched/fair: Fix potential memory corruption in child_cfs_rq_on_list

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/5cb300dcdd27e6a351ac02541e0231261c775852
• https://git.kernel.org/stable/c/000c9ee43928f2ce68a156dd40bab7616256f4dd
• https://git.kernel.org/stable/c/9cc7f0018609f75a349e42e3aebc3b0e905ba775
• https://git.kernel.org/stable/c/b5741e4b9ef3567613b2351384f91d3f16e59986
• https://git.kernel.org/stable/c/e1dd09df30ba86716cb2ffab97dc35195c01eb8f
• https://git.kernel.org/stable/c/3b4035ddbfc8e4521f85569998a7569668cccf51
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html