GHSA-JQ2V-J4FW-M4MG
Vulnerability from github – Published: 2025-05-08 09:30 – Updated: 2025-11-12 21:31In the Linux kernel, the following vulnerability has been resolved:
sched/eevdf: Fix se->slice being set to U64_MAX and resulting crash
There is a code path in dequeue_entities() that can set the slice of a sched_entity to U64_MAX, which sometimes results in a crash.
The offending case is when dequeue_entities() is called to dequeue a delayed group entity, and then the entity's parent's dequeue is delayed. In that case:
- In the if (entity_is_task(se)) else block at the beginning of dequeue_entities(), slice is set to cfs_rq_min_slice(group_cfs_rq(se)). If the entity was delayed, then it has no queued tasks, so cfs_rq_min_slice() returns U64_MAX.
- The first for_each_sched_entity() loop dequeues the entity.
- If the entity was its parent's only child, then the next iteration tries to dequeue the parent.
- If the parent's dequeue needs to be delayed, then it breaks from the first for_each_sched_entity() loop without updating slice.
- The second for_each_sched_entity() loop sets the parent's ->slice to the saved slice, which is still U64_MAX.
This throws off subsequent calculations with potentially catastrophic results. A manifestation we saw in production was:
- In update_entity_lag(), se->slice is used to calculate limit, which ends up as a huge negative number.
- limit is used in se->vlag = clamp(vlag, -limit, limit). Because limit is negative, vlag > limit, so se->vlag is set to the same huge negative number.
- In place_entity(), se->vlag is scaled, which overflows and results in another huge (positive or negative) number.
- The adjusted lag is subtracted from se->vruntime, which increases or decreases se->vruntime by a huge number.
- pick_eevdf() calls entity_eligible()/vruntime_eligible(), which incorrectly returns false because the vruntime is so far from the other vruntimes on the queue, causing the (vruntime - cfs_rq->min_vruntime) * load calulation to overflow.
- Nothing appears to be eligible, so pick_eevdf() returns NULL.
- pick_next_entity() tries to dereference the return value of pick_eevdf() and crashes.
Dumping the cfs_rq states from the core dumps with drgn showed tell-tale huge vruntime ranges and bogus vlag values, and I also traced se->slice being set to U64_MAX on live systems (which was usually "benign" since the rest of the runqueue needed to be in a particular state to crash).
Fix it in dequeue_entities() by always setting slice from the first non-empty cfs_rq.
{
"affected": [],
"aliases": [
"CVE-2025-37821"
],
"database_specific": {
"cwe_ids": [
"CWE-476"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-05-08T07:15:53Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsched/eevdf: Fix se-\u003eslice being set to U64_MAX and resulting crash\n\nThere is a code path in dequeue_entities() that can set the slice of a\nsched_entity to U64_MAX, which sometimes results in a crash.\n\nThe offending case is when dequeue_entities() is called to dequeue a\ndelayed group entity, and then the entity\u0027s parent\u0027s dequeue is delayed.\nIn that case:\n\n1. In the if (entity_is_task(se)) else block at the beginning of\n dequeue_entities(), slice is set to\n cfs_rq_min_slice(group_cfs_rq(se)). If the entity was delayed, then\n it has no queued tasks, so cfs_rq_min_slice() returns U64_MAX.\n2. The first for_each_sched_entity() loop dequeues the entity.\n3. If the entity was its parent\u0027s only child, then the next iteration\n tries to dequeue the parent.\n4. If the parent\u0027s dequeue needs to be delayed, then it breaks from the\n first for_each_sched_entity() loop _without updating slice_.\n5. The second for_each_sched_entity() loop sets the parent\u0027s -\u003eslice to\n the saved slice, which is still U64_MAX.\n\nThis throws off subsequent calculations with potentially catastrophic\nresults. A manifestation we saw in production was:\n\n6. In update_entity_lag(), se-\u003eslice is used to calculate limit, which\n ends up as a huge negative number.\n7. limit is used in se-\u003evlag = clamp(vlag, -limit, limit). Because limit\n is negative, vlag \u003e limit, so se-\u003evlag is set to the same huge\n negative number.\n8. In place_entity(), se-\u003evlag is scaled, which overflows and results in\n another huge (positive or negative) number.\n9. The adjusted lag is subtracted from se-\u003evruntime, which increases or\n decreases se-\u003evruntime by a huge number.\n10. pick_eevdf() calls entity_eligible()/vruntime_eligible(), which\n incorrectly returns false because the vruntime is so far from the\n other vruntimes on the queue, causing the\n (vruntime - cfs_rq-\u003emin_vruntime) * load calulation to overflow.\n11. Nothing appears to be eligible, so pick_eevdf() returns NULL.\n12. pick_next_entity() tries to dereference the return value of\n pick_eevdf() and crashes.\n\nDumping the cfs_rq states from the core dumps with drgn showed tell-tale\nhuge vruntime ranges and bogus vlag values, and I also traced se-\u003eslice\nbeing set to U64_MAX on live systems (which was usually \"benign\" since\nthe rest of the runqueue needed to be in a particular state to crash).\n\nFix it in dequeue_entities() by always setting slice from the first\nnon-empty cfs_rq.",
"id": "GHSA-jq2v-j4fw-m4mg",
"modified": "2025-11-12T21:31:01Z",
"published": "2025-05-08T09:30:24Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-37821"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/50a665496881262519f115f1bfe5822f30580eb0"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/86b37810fa1e40b93171da023070b99ccbb4ea04"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/bbce3de72be56e4b5f68924b7da9630cc89aa1a8"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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