{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21919",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-21919"
  ],
  "published": "2025-04-01T15:40:16Z",
  "summary": "sched/fair: Fix potential memory corruption in child_cfs_rq_on_list",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsched/fair: Fix potential memory corruption in child_cfs_rq_on_list\n\nchild_cfs_rq_on_list attempts to convert a 'prev' pointer to a cfs_rq.\nThis 'prev' pointer can originate from struct rq's leaf_cfs_rq_list,\nmaking the conversion invalid and potentially leading to memory\ncorruption. Depending on the relative positions of leaf_cfs_rq_list and\nthe task group (tg) pointer within the struct, this can cause a memory\nfault or access garbage data.\n\nThe issue arises in list_add_leaf_cfs_rq, where both\ncfs_rq->leaf_cfs_rq_list and rq->leaf_cfs_rq_list are added to the same\nleaf list. Also, rq->tmp_alone_branch can be set to rq->leaf_cfs_rq_list.\n\nThis adds a check `if (prev == &rq->leaf_cfs_rq_list)` after the main\nconditional in child_cfs_rq_on_list. This ensures that the container_of\noperation will convert a correct cfs_rq struct.\n\nThis check is sufficient because only cfs_rqs on the same CPU are added\nto the list, so verifying the 'prev' pointer against the current rq's list\nhead is enough.\n\nFixes a potential memory corruption issue that due to current struct\nlayout might not be manifesting as a crash but could lead to unpredictable\nbehavior when the layout changes.",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21919"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/5cb300dcdd27e6a351ac02541e0231261c775852"
    },
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      "url": "https://git.kernel.org/stable/c/000c9ee43928f2ce68a156dd40bab7616256f4dd"
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    {
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    "upstream_modified": "2026-08-05T12:19:11Z",
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      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The defect is in core CFS scheduler code (`kernel/sched/fair.c`) reached through ordinary local scheduling activity, cgroup manipulation, and CFS-bandwidth unthrottling; there is no network-facing input path. Exploitation requires the ability to run code on the machine.\nAC:L - The invalid `container_of()` executes deterministically every time the examined cfs_rq is first on `rq->leaf_cfs_rq_list` or `rq->tmp_alone_branch` is at its reset value, which is the normal steady state \u2014 no race to win and no unusual precondition. An attacker reliably drives it by creating/loading a cgroup in a delegated subtree and letting the load balancer or bandwidth timer run.\nPR:L - No capability check, credential test, or privileged gate exists on `__update_blocked_fair()` or `tg_unthrottle_up()`; an ordinary unprivileged user reaches both via normal task execution and via cgroup v2 cpu/cpu.max controls in a systemd- or container-delegated subtree. Real root in the init namespace is not needed.\nUI:N - The vulnerable path runs from the periodic load-balance softirq and the CFS bandwidth timer, with no victim action of any kind required. The attacker's own workload is sufficient to trigger it.\nS:U - The bogus pointer arithmetic, out-of-bounds read, and any resulting fault all occur within the kernel's own security authority on the same host. No VM, IOMMU, or sandbox boundary is crossed.\nC:H - The type-confused `struct task_group *` is dereferenced at `offsetof(task_group, parent)` from a `list_head` address, which reads out of bounds past the end of the `cfs_rq` slab object into adjacent heap memory whose contents an attacker can influence via heap grooming. This is an unbounded kernel-memory read at an offset determined by struct layout rather than by any bounds check.\nI:H - This is a genuine type confusion \u2014 a `struct rq` list head reinterpreted as a member of `struct cfs_rq`, yielding a fabricated object pointer that is dereferenced \u2014 which upstream explicitly characterizes as \"potential memory corruption.\" Under differing struct layouts the fabricated `tg` pointer is an arbitrary non-pointer value, so the corrupted access is not confined to a benign read.\nA:H - The commit states the invalid conversion \"can cause a memory fault or access garbage data\"; dereferencing a wild `tg` pointer inside the scheduler with the rq lock held produces an oops or panic, and the slab-out-of-bounds read triggers a KASAN report that panics on hardened configurations. A fault in `__update_blocked_fair()` under the runqueue lock is unrecoverable for the affected CPU."
    }
  }
}