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CVE Record

CVE-2021-47566: proc/vmcore: fix clearing user buffer by properly using clear_user()

In the Linux kernel, the following vulnerability has been resolved: proc/vmcore: fix clearing user buffer by properly using clear_user() To clear a user buffer we cannot simply use memset, we have to use clear_user(). With a virtio-mem device that registers a vmcore_cb and has some logically unplugged memory inside an added Linux memory block, I can easily trigger a BUG by copying the vmcore via "cp": systemd[1]: Starting Kdump Vmcore Save Service... kdump[420]: Kdump is using the default log level(3). kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/ kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/ kdump[465]: saving vmcore-dmesg.txt complete kdump[467]: saving vmcore BUG: unable to handle page fault for address: 00007f2374e01000 #PF: supervisor write access in kernel mode #PF: error_code(0x0003) - permissions violation PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867 Oops: 0003 [#1] PREEMPT SMP NOPTI CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014 RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86 Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7 38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 <49> c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81 RSP: 0018:ffffc9000073be08 EFLAGS: 00010212 RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000 RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008 RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50 R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000 R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8 FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0 Call Trace: read_vmcore+0x236/0x2c0 proc_reg_read+0x55/0xa0 vfs_read+0x95/0x190 ksys_read+0x4f/0xc0 do_syscall_64+0x3b/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access Prevention (SMAP)", which is used to detect wrong access from the kernel to user buffers like this: SMAP triggers a permissions violation on wrong access. In the x86-64 variant of clear_user(), SMAP is properly handled via clac()+stac(). To fix, properly use clear_user() when we're dealing with a user buffer.

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 the system while saving a crash dump in a specific virtio-mem and vmcore scenario. It affects reliability of post-crash evidence collection more than ordinary application security. The sources do not show remote exploitation, privilege escalation, or confirmed active exploitation.

Executive priority

Treat as a targeted reliability and forensic-collection risk, not an internet-facing emergency based on current evidence. Patch through normal kernel maintenance, with higher priority for virtualized or memory-hotplug environments relying on crash dumps for incident response.

Technical view

The vmcore read path cleared a user buffer with memset instead of clear_user. On x86-64 systems with SMAP, that wrong supervisor write to user memory can trigger a page fault and kernel BUG during read_vmcore/proc_reg_read. Stable kernel commits replace this with clear_user for user-buffer handling.

Likely exposure

Most relevant exposure is Linux systems using kdump or /proc/vmcore with virtio-mem, especially where logically unplugged memory exists inside an added memory block. General Linux exposure should be validated against vendor kernel versions and backports.

Exploitation context

The provided evidence describes a local crash-dump copy path triggering a BUG. CISA KEV status is false in the bundle, and no cited source claims active exploitation. Evidence is incomplete for broader exploitability beyond the described vmcore scenario.

Researcher notes

The source narrative is precise but narrow: virtio-mem vmcore_cb, logically unplugged memory, SMAP, and copying vmcore. No CVSS, CWE, or exploit confirmation is provided. Avoid broad claims until vendor advisories or kernel maintainers document wider impact.

Mitigation direction

  • Apply the relevant Linux stable kernel fix or vendor backport.
  • Prioritize hosts using kdump, /proc/vmcore, and virtio-mem.
  • Check Linux distribution advisories for patched package versions.
  • Avoid direct wrangler-style assumptions; verify kernel provenance and backport status.
  • If patching is delayed, review vendor guidance for kdump or virtio-mem risk reduction.

Validation and detection

  • Inventory Linux kernel versions and distribution backport levels.
  • Identify systems using kdump, /proc/vmcore, or virtio-mem.
  • Confirm the vmcore clear_user fix exists in the running kernel source or changelog.
  • Review crash-dump logs for BUG/Oops events in read_vmcore or read_from_oldmem.
  • Track remediation through vendor advisory and kernel package deployment records.
Prepared
Confidence
medium
Sources
10

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-2021-47566 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
9Source 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
LinuxLinux997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90, 997c136f518c5debd63847e78e2a8694f56dcf90unaffected
LinuxLinux3.0, 0, 4.4.294, 4.9.292, 4.14.257, 4.19.219, 5.4.163, 5.10.83, 5.15.6, 5.16affected
Weakness

CWE details

No CWE listed

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