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

• Severity: high (CVSS 7.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-04-01 · updated 2026-10-15
• Weakness: CWE-787

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 products

• Linux kernel: from 5.13 before 5.15.179; from 5.16 before 6.1.131; from 6.2 before 6.6.83; from 6.7 before 6.12.19; from 6.13 before 6.13.7; 6.14-rc1, 6.14-rc2, 6.14-rc3, 6.14-rc4, 6.14-rc5 — fixed in 5.15.179, 6.1.131, 6.12.19, 6.13.7, 6.6.83

How to fix

Upgrade to a fixed version: Linux kernel 5.15.179, 6.1.131, 6.12.19, 6.13.7. Check the vendor advisory for mitigations.

References

• https://git.kernel.org/stable/c/000c9ee43928f2ce68a156dd40bab7616256f4dd
• https://git.kernel.org/stable/c/3b4035ddbfc8e4521f85569998a7569668cccf51
• https://git.kernel.org/stable/c/5cb300dcdd27e6a351ac02541e0231261c775852
• https://git.kernel.org/stable/c/9cc7f0018609f75a349e42e3aebc3b0e905ba775
• https://git.kernel.org/stable/c/b5741e4b9ef3567613b2351384f91d3f16e59986
• https://git.kernel.org/stable/c/e1dd09df30ba86716cb2ffab97dc35195c01eb8f
• 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

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