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CVE Record

CVE-2025-21655: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

In the Linux kernel, the following vulnerability has been resolved: io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period io_eventfd_do_signal() is invoked from an RCU callback, but when dropping the reference to the io_ev_fd, it calls io_eventfd_free() directly if the refcount drops to zero. This isn't correct, as any potential freeing of the io_ev_fd should be deferred another RCU grace period. Just call io_eventfd_put() rather than open-code the dec-and-test and free, which will correctly defer it another RCU grace period.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This Linux kernel flaw mishandles when an io_uring event-notification object may be freed. A local user with limited privileges could potentially trigger unsafe kernel memory handling, with the supplied CVSS assessment allowing serious confidentiality, integrity, and availability impact. The supplied vector rates the attack as local and requiring no victim interaction.

Executive priority

Treat this as high priority on multi-user Linux systems and environments executing untrusted local workloads. Patch through supported vendor kernels after confirming package-level exposure. Internet exposure alone does not establish reachability, but existing local access or hostile workloads materially increase urgency.

Technical view

An io_uring eventfd RCU callback could drop the last io_ev_fd reference and call io_eventfd_free() immediately. This can free the object without the additional RCU grace period required for concurrent readers. The correction routes release through io_eventfd_put(), which performs the deferred free. CVSS 3.1 is 7.8: local, low complexity, low privileges, and no interaction.

Likely exposure

Exposure is most relevant on Linux hosts where low-privileged local users or workloads can reach the affected io_uring/eventfd path. The bundle lists 6.1, 6.1.125, 6.6.72, 6.12.10, and 6.13, but its version metadata is ambiguous. Verify distribution packages and backports instead of relying solely on version strings.

Exploitation context

CISA KEV status is false, and none of the supplied evidence states active exploitation. A Project Zero issue is referenced, but the bundle provides no exploitability details. That reference alone does not prove public exploitation or in-the-wild attacks.

Researcher notes

No CWE is assigned in the bundle. The central issue is RCU lifetime management rather than eventfd permission checking. Downstream version strings may not prove exposure because distributors backport fixes. The supplied version data also contains duplicate commit entries and an anomalous "0", reducing confidence in exact range interpretation.

Mitigation direction

  • Install a vendor-supported kernel containing the upstream stable fix, then reboot affected systems.
  • Use relevant vendor advisories to identify fixed distribution or product package releases.
  • Prioritize multi-user systems and workloads permitting untrusted local code execution.
  • If patching is delayed, follow vendor-provided temporary mitigations; none are specified in the supplied evidence.

Validation and detection

  • Inventory running kernel versions and distribution package revisions across Linux hosts.
  • Map downstream packages to the listed stable commits or vendor-confirmed backports.
  • After updating and rebooting, verify the running kernel is the intended fixed build.
  • Review kernel fault telemetry for events mentioning io_uring or eventfd.
Prepared
Confidence
medium
Sources
9

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

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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CVE-2025-21655 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

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.

1CVSS vectors
3Timeline events
2ADP providers
8Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2025-21655Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

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

CVECVE Program Container
siemens-SADPADP container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux21a091b970cdbcf3e8ff829234b51be6f9192766, 21a091b970cdbcf3e8ff829234b51be6f9192766, 21a091b970cdbcf3e8ff829234b51be6f9192766, 21a091b970cdbcf3e8ff829234b51be6f9192766unaffected
LinuxLinux6.1, 0, 6.1.125, 6.6.72, 6.12.10, 6.13affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.