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CVE Record

CVE-2021-47256: mm/memory-failure: make sure wait for page writeback in memory_failure

In the Linux kernel, the following vulnerability has been resolved: mm/memory-failure: make sure wait for page writeback in memory_failure Our syzkaller trigger the "BUG_ON(!list_empty(&inode->i_wb_list))" in clear_inode: kernel BUG at fs/inode.c:519! Internal error: Oops - BUG: 0 [#1] SMP Modules linked in: Process syz-executor.0 (pid: 249, stack limit = 0x00000000a12409d7) CPU: 1 PID: 249 Comm: syz-executor.0 Not tainted 4.19.95 Hardware name: linux,dummy-virt (DT) pstate: 80000005 (Nzcv daif -PAN -UAO) pc : clear_inode+0x280/0x2a8 lr : clear_inode+0x280/0x2a8 Call trace: clear_inode+0x280/0x2a8 ext4_clear_inode+0x38/0xe8 ext4_free_inode+0x130/0xc68 ext4_evict_inode+0xb20/0xcb8 evict+0x1a8/0x3c0 iput+0x344/0x460 do_unlinkat+0x260/0x410 __arm64_sys_unlinkat+0x6c/0xc0 el0_svc_common+0xdc/0x3b0 el0_svc_handler+0xf8/0x160 el0_svc+0x10/0x218 Kernel panic - not syncing: Fatal exception A crash dump of this problem show that someone called __munlock_pagevec to clear page LRU without lock_page: do_mmap -> mmap_region -> do_munmap -> munlock_vma_pages_range -> __munlock_pagevec. As a result memory_failure will call identify_page_state without wait_on_page_writeback. And after truncate_error_page clear the mapping of this page. end_page_writeback won't call sb_clear_inode_writeback to clear inode->i_wb_list. That will trigger BUG_ON in clear_inode! Fix it by checking PageWriteback too to help determine should we skip wait_on_page_writeback.

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 vulnerable system when the memory-failure handler mishandles a page that is still being written back. The public record describes a kernel panic found by syzkaller, not data theft or remote compromise. Business urgency depends on whether affected kernel builds are used on production or multi-tenant systems.

Executive priority

Treat as a reliability and availability issue. It should be scheduled into normal kernel patching, with faster handling for shared infrastructure or systems running untrusted workloads. Current sources do not justify emergency response based on exploitation evidence.

Technical view

The bug is in mm/memory-failure handling. The fix adds PageWriteback consideration so memory_failure waits appropriately before truncate_error_page clears page mapping. Without that wait, end_page_writeback may not clear inode writeback state, leading to BUG_ON in clear_inode and kernel panic.

Likely exposure

Exposure is likely limited to systems running affected Linux kernel versions or downstream builds without the stable fix. The bundle lists affected Linux versions including 3.16, 4.14.238, 4.19.196, 5.4.128, 5.10.46, 5.12.13, and 5.13; distribution backports must be verified separately.

Exploitation context

No active exploitation is supported by the provided sources. KEV is false. The evidence describes a syzkaller-triggered crash path and kernel panic. The bundle does not identify remote exploitation, privilege escalation, public weaponization, or confirmed real-world abuse.

Researcher notes

The source evidence is kernel-resolution text plus stable commit references. Impact is demonstrated as a kernel panic from incorrect writeback waiting in memory_failure. Affected-version metadata is broad and should be reconciled with distro backports before declaring individual hosts vulnerable.

Mitigation direction

  • Apply vendor kernel updates that include the referenced stable fixes.
  • Verify distribution backports rather than relying only on upstream version numbers.
  • Prioritize production, multi-tenant, and untrusted-code Linux hosts.
  • If patching is delayed, check vendor guidance for supported mitigations.

Validation and detection

  • Inventory running kernel versions across Linux hosts.
  • Confirm whether the relevant stable commits or vendor backports are present.
  • Review crash logs for clear_inode, inode i_wb_list, or memory_failure panic indicators.
  • Check CVE tracking against distribution advisories for each deployed kernel line.
Prepared
Confidence
medium
Sources
8

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-2021-47256 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
7Source 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

CVECVE Program Container
CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux0bc1f8b0682caa39f45ce1e0228ebf43acb46111, 0bc1f8b0682caa39f45ce1e0228ebf43acb46111, 0bc1f8b0682caa39f45ce1e0228ebf43acb46111, 0bc1f8b0682caa39f45ce1e0228ebf43acb46111, 0bc1f8b0682caa39f45ce1e0228ebf43acb46111, 0bc1f8b0682caa39f45ce1e0228ebf43acb46111unaffected
LinuxLinux3.16, 0, 4.14.238, 4.19.196, 5.4.128, 5.10.46, 5.12.13, 5.13affected
Weakness

CWE details

No CWE listed

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