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CVE Record

CVE-2021-47584: iocost: Fix divide-by-zero on donation from low hweight cgroup

In the Linux kernel, the following vulnerability has been resolved: iocost: Fix divide-by-zero on donation from low hweight cgroup The donation calculation logic assumes that the donor has non-zero after-donation hweight, so the lowest active hweight a donating cgroup can have is 2 so that it can donate 1 while keeping the other 1 for itself. Earlier, we only donated from cgroups with sizable surpluses so this condition was always true. However, with the precise donation algorithm implemented, f1de2439ec43 ("blk-iocost: revamp donation amount determination") made the donation amount calculation exact enabling even low hweight cgroups to donate. This means that in rare occasions, a cgroup with active hweight of 1 can enter donation calculation triggering the following warning and then a divide-by-zero oops. WARNING: CPU: 4 PID: 0 at block/blk-iocost.c:1928 transfer_surpluses.cold+0x0/0x53 [884/94867] ... RIP: 0010:transfer_surpluses.cold+0x0/0x53 Code: 92 ff 48 c7 c7 28 d1 ab b5 65 48 8b 34 25 00 ae 01 00 48 81 c6 90 06 00 00 e8 8b 3f fe ff 48 c7 c0 ea ff ff ff e9 95 ff 92 ff <0f> 0b 48 c7 c7 30 da ab b5 e8 71 3f fe ff 4c 89 e8 4d 85 ed 74 0 4 ... Call Trace: <IRQ> ioc_timer_fn+0x1043/0x1390 call_timer_fn+0xa1/0x2c0 __run_timers.part.0+0x1ec/0x2e0 run_timer_softirq+0x35/0x70 ... iocg: invalid donation weights in /a/b: active=1 donating=1 after=0 Fix it by excluding cgroups w/ active hweight < 2 from donating. Excluding these extreme low hweight donations shouldn't affect work conservation in any meaningful way.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This Linux kernel flaw can crash a system under a rare block I/O control-group accounting condition. The issue is a divide-by-zero in the iocost donation logic when a very low-weight cgroup donates all remaining weight. Business impact is primarily availability risk, not confirmed data theft or privilege escalation.

Executive priority

Treat as a planned availability patch unless affected hosts are business-critical or heavily use cgroup I/O controls. There is no provided evidence of active exploitation, but a kernel crash can still create operational disruption.

Technical view

In blk-iocost, precise donation calculation allowed a cgroup with active hweight 1 to enter donation logic, leaving after-donation hweight 0 and triggering a divide-by-zero oops. The kernel fix excludes cgroups with active hweight below 2 from donating. Sources do not provide CVSS, CWE, or exploitability details.

Likely exposure

Exposure is most relevant to Linux systems using cgroups and the blk-iocost controller. The bundle identifies Linux kernel versions around 5.10, 5.15, and 5.16, but version-range evidence is incomplete and should be checked against distribution advisories.

Exploitation context

The source bundle does not show active exploitation, and KEV status is false. The described failure happens in rare low-weight cgroup donation conditions and results in a warning followed by a divide-by-zero kernel oops.

Researcher notes

The root condition is active hweight below 2 entering donation calculation after commit f1de2439ec43 changed donation determination. The public description names the fix behavior but does not provide CVSS, CWE mapping, exploit prerequisites, or distribution-specific package status.

Mitigation direction

  • Apply vendor or distribution kernel updates containing the referenced stable fixes.
  • Check Linux distribution advisories for exact affected and fixed package versions.
  • Prioritize systems using cgroup-based block I/O control or container workloads.
  • If patching is delayed, review vendor guidance for operational workarounds.

Validation and detection

  • Inventory Linux kernel versions across servers, hosts, and container platforms.
  • Check whether blk-iocost and cgroup I/O control are enabled or used.
  • Review kernel logs for iocost warnings or divide-by-zero oops messages.
  • Confirm installed kernels include the relevant upstream stable fix commits.
Prepared
Confidence
medium
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
2ADP providers
4Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxf1de2439ec43b74764f2a26e3a310b24407e3bde, f1de2439ec43b74764f2a26e3a310b24407e3bde, f1de2439ec43b74764f2a26e3a310b24407e3bdeunaffected
LinuxLinux5.10, 0, 5.10.88, 5.15.11, 5.16affected
Weakness

CWE details

No CWE listed

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