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CVE Record

CVE-2021-46988: userfaultfd: release page in error path to avoid BUG_ON

In the Linux kernel, the following vulnerability has been resolved: userfaultfd: release page in error path to avoid BUG_ON Consider the following sequence of events: 1. Userspace issues a UFFD ioctl, which ends up calling into shmem_mfill_atomic_pte(). We successfully account the blocks, we shmem_alloc_page(), but then the copy_from_user() fails. We return -ENOENT. We don't release the page we allocated. 2. Our caller detects this error code, tries the copy_from_user() after dropping the mmap_lock, and retries, calling back into shmem_mfill_atomic_pte(). 3. Meanwhile, let's say another process filled up the tmpfs being used. 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and immediately returns - without releasing the page. This triggers a BUG_ON in our caller, which asserts that the page should always be consumed, unless -ENOENT is returned. To fix this, detect if we have such a "dangling" page when accounting fails, and if so, release it before returning.

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 issue can crash a system when a userfaultfd operation hits a specific error path involving shared memory. The public record describes a page-reference cleanup bug leading to a BUG_ON assertion. No CVSS score or active exploitation evidence is provided.

Executive priority

Treat as a routine but real availability fix for Linux fleets. Escalate priority for shared hosting, CI runners, container platforms, or any system where local users are not fully trusted.

Technical view

The flaw is in userfaultfd handling via shmem_mfill_atomic_pte(). A failed copy_from_user() can leave an allocated page dangling, then a retry after tmpfs accounting failure returns without consuming or releasing it, triggering a caller BUG_ON. Stable kernel commits release the page on that error path.

Likely exposure

Exposure is limited to Linux systems running affected kernel versions listed in the record, especially hosts where untrusted local workloads can use userfaultfd and tmpfs-backed memory. The bundle does not identify affected distributions or cloud images.

Exploitation context

The sources do not report active exploitation, and CVE data says this is not in KEV. The described impact is a kernel BUG_ON from a local kernel path, suggesting availability risk rather than confirmed remote compromise.

Researcher notes

The public details describe a race-like error-path cleanup problem, not a complete exploit chain. Validation should focus on kernel lineage, vendor backports, userfaultfd exposure, and tmpfs/shared-memory use. Do not infer privilege escalation without additional evidence.

Mitigation direction

  • Update to a vendor kernel containing the listed stable fixes.
  • Prioritize multi-tenant hosts and systems running untrusted local workloads.
  • Check distribution advisories for exact package versions and backports.
  • If updates are delayed, review vendor guidance for userfaultfd restrictions.

Validation and detection

  • Inventory Linux kernel versions against the affected versions in the CVE record.
  • Confirm installed kernel packages include vendor backports for the referenced commits.
  • Review whether untrusted users or containers can access userfaultfd functionality.
  • Verify patch status through distribution security advisories, not commit hashes alone.
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.

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CVE-2021-46988 mapping review

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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
8Source 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
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564, cb658a453b9327ce96ce5222c24d162b5b65b564unaffected
LinuxLinux4.11, 0, 4.14.233, 4.19.191, 5.4.120, 5.10.38, 5.11.22, 5.12.5, 5.13affected
Weakness

CWE details

No CWE listed

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