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CVE Record

CVE-2021-47089: kfence: fix memory leak when cat kfence objects

In the Linux kernel, the following vulnerability has been resolved: kfence: fix memory leak when cat kfence objects Hulk robot reported a kmemleak problem: unreferenced object 0xffff93d1d8cc02e8 (size 248): comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s) hex dump (first 32 bytes): 00 40 85 19 d4 93 ff ff 00 10 00 00 00 00 00 00 .@.............. 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ backtrace: seq_open+0x2a/0x80 full_proxy_open+0x167/0x1e0 do_dentry_open+0x1e1/0x3a0 path_openat+0x961/0xa20 do_filp_open+0xae/0x120 do_sys_openat2+0x216/0x2f0 do_sys_open+0x57/0x80 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 unreferenced object 0xffff93d419854000 (size 4096): comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s) hex dump (first 32 bytes): 6b 66 65 6e 63 65 2d 23 32 35 30 3a 20 30 78 30 kfence-#250: 0x0 30 30 30 30 30 30 30 37 35 34 62 64 61 31 32 2d 0000000754bda12- backtrace: seq_read_iter+0x313/0x440 seq_read+0x14b/0x1a0 full_proxy_read+0x56/0x80 vfs_read+0xa5/0x1b0 ksys_read+0xa0/0xf0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 I find that we can easily reproduce this problem with the following commands: cat /sys/kernel/debug/kfence/objects echo scan > /sys/kernel/debug/kmemleak cat /sys/kernel/debug/kmemleak The leaked memory is allocated in the stack below: do_syscall_64 do_sys_open do_dentry_open full_proxy_open seq_open ---> alloc seq_file vfs_read full_proxy_read seq_read seq_read_iter traverse ---> alloc seq_buf And it should have been released in the following process: do_syscall_64 syscall_exit_to_user_mode exit_to_user_mode_prepare task_work_run ____fput __fput full_proxy_release ---> free here However, the release function corresponding to file_operations is not implemented in kfence. As a result, a memory leak occurs. Therefore, the solution to this problem is to implement the corresponding release function.

LowCVSS 3.3Not KEV-listedUpdated
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Glexia's TakeAutomated analysislow

Security readout for executives and security teams

Plain-English summary

This is a low-severity Linux kernel memory leak in the KFENCE debug interface. A local user able to access that interface could cause unreleased kernel memory after reading KFENCE objects, potentially reducing availability over time. The sources do not indicate data theft, integrity impact, or known active exploitation.

Executive priority

Treat as routine patching unless you operate shared Linux systems with untrusted local users. Business risk is mainly minor availability degradation, not compromise of sensitive data, based on the supplied sources.

Technical view

KFENCE lacked the release handler corresponding to its file operations. Reading the KFENCE objects debug interface could allocate seq_file and seq_buf memory that was not freed on file close. The kernel fix implements the missing release path. CVSS is 3.3, local attack vector, low availability impact only.

Likely exposure

Exposure is limited to Linux systems running affected kernel builds with KFENCE and its debug interface accessible to a local low-privileged user. The source bundle names Linux kernel versions around 5.12 through fixed 5.15.12/5.16 lines, but exact vendor package mappings require distributor advisories.

Exploitation context

The CVE is not listed as CISA KEV in the provided bundle, and no cited source reports active exploitation. Abuse requires local access and interaction with a kernel debug interface, making broad remote exploitation unlikely from the available evidence.

Researcher notes

The public description ties the leak to missing KFENCE file_operations release handling after seq_file and seq_buf allocation. Evidence supports memory leak and low availability impact only. Avoid assuming exploitability beyond local debug interface access without vendor-specific configuration details.

Mitigation direction

  • Apply Linux kernel updates containing the referenced stable fixes.
  • Check your Linux distribution advisory for exact fixed package versions.
  • Restrict access to debugfs and KFENCE debug interfaces where not operationally needed.
  • Prioritize shared multi-user systems where local users are untrusted.

Validation and detection

  • Inventory Linux kernel versions and compare against vendor fixed versions.
  • Confirm whether KFENCE and debugfs are enabled on exposed hosts.
  • Check whether local non-admin users can access KFENCE debug interfaces.
  • Review kernel update status against the two referenced stable commits.
Prepared
Confidence
high
Sources
4

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

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CVE-2021-47089 mapping review

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Vulnerability profileCVE Program record
Severity
Low
CVSS
3.3 (3.1)
Known Exploited
No
Published

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

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
3Source links

SSVC decision data

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

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
3.3CVSS 3.1LowCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L1.81.4CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

3.3Low
CVSS 3.1 vector shape for CVE-2021-47089Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

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

CISA-ADPCISA ADP Vulnrichment
cvssV3_1other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux0ce20dd840897b12ae70869c69f1ba34d6d16965, 0ce20dd840897b12ae70869c69f1ba34d6d16965unaffected
LinuxLinux5.12, 0, 5.15.12, 5.16affected
Weakness

CWE details

No CWE listed

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