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CVE Record

CVE-2021-46984: kyber: fix out of bounds access when preempted

In the Linux kernel, the following vulnerability has been resolved: kyber: fix out of bounds access when preempted __blk_mq_sched_bio_merge() gets the ctx and hctx for the current CPU and passes the hctx to ->bio_merge(). kyber_bio_merge() then gets the ctx for the current CPU again and uses that to get the corresponding Kyber context in the passed hctx. However, the thread may be preempted between the two calls to blk_mq_get_ctx(), and the ctx returned the second time may no longer correspond to the passed hctx. This "works" accidentally most of the time, but it can cause us to read garbage if the second ctx came from an hctx with more ctx's than the first one (i.e., if ctx->index_hw[hctx->type] > hctx->nr_ctx). This manifested as this UBSAN array index out of bounds error reported by Jakub: UBSAN: array-index-out-of-bounds in ../kernel/locking/qspinlock.c:130:9 index 13106 is out of range for type 'long unsigned int [128]' Call Trace: dump_stack+0xa4/0xe5 ubsan_epilogue+0x5/0x40 __ubsan_handle_out_of_bounds.cold.13+0x2a/0x34 queued_spin_lock_slowpath+0x476/0x480 do_raw_spin_lock+0x1c2/0x1d0 kyber_bio_merge+0x112/0x180 blk_mq_submit_bio+0x1f5/0x1100 submit_bio_noacct+0x7b0/0x870 submit_bio+0xc2/0x3a0 btrfs_map_bio+0x4f0/0x9d0 btrfs_submit_data_bio+0x24e/0x310 submit_one_bio+0x7f/0xb0 submit_extent_page+0xc4/0x440 __extent_writepage_io+0x2b8/0x5e0 __extent_writepage+0x28d/0x6e0 extent_write_cache_pages+0x4d7/0x7a0 extent_writepages+0xa2/0x110 do_writepages+0x8f/0x180 __writeback_single_inode+0x99/0x7f0 writeback_sb_inodes+0x34e/0x790 __writeback_inodes_wb+0x9e/0x120 wb_writeback+0x4d2/0x660 wb_workfn+0x64d/0xa10 process_one_work+0x53a/0xa80 worker_thread+0x69/0x5b0 kthread+0x20b/0x240 ret_from_fork+0x1f/0x30 Only Kyber uses the hctx, so fix it by passing the request_queue to ->bio_merge() instead. BFQ and mq-deadline just use that, and Kyber can map the queues itself to avoid the mismatch.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is a Linux kernel storage scheduler bug in Kyber. Under a timing race during CPU preemption, the kernel could use mismatched queue context and read outside expected memory bounds. The public bundle shows a kernel fix but no CVSS score, no KEV listing, and no cited active exploitation.

Executive priority

Treat as a normal kernel maintenance priority unless local risk is elevated by Kyber usage or sensitive multi-tenant workloads. There is no source-backed evidence of active exploitation, but kernel memory safety bugs should not remain unpatched indefinitely.

Technical view

__blk_mq_sched_bio_merge() passed an hctx derived from one CPU context, while kyber_bio_merge() recomputed ctx later. If preempted between those operations, ctx could map to a different hctx, causing an out-of-bounds array access. The fix changes bio_merge handling so Kyber maps queues consistently from the request_queue.

Likely exposure

Exposure is limited to Linux systems running affected kernel versions with the Kyber I/O scheduler path reachable. The bundle identifies Linux as affected and references stable kernel fixes, but distro-specific package mappings are not provided.

Exploitation context

No active exploitation is supported by the supplied sources, and the CVE is not marked KEV. Evidence describes a kernel UBSAN out-of-bounds report during filesystem writeback, not a public exploit or reliable remote attack path.

Researcher notes

The key condition is preemption between two ctx lookups, producing hctx/ctx mismatch. The bundle does not provide CVSS, CWE, exploitability analysis, or distro advisories. Validate impact against exact kernel backports rather than upstream version numbers alone.

Mitigation direction

  • Apply vendor kernel updates containing the referenced stable fixes.
  • Check Linux distribution advisories for exact fixed package versions.
  • Prioritize systems using Kyber or affected block multiqueue scheduler paths.
  • If patch timing is constrained, review vendor guidance for scheduler workarounds.

Validation and detection

  • Inventory Linux kernel versions across servers and appliances.
  • Determine whether Kyber is configured or selectable on exposed systems.
  • Map installed distro kernel packages to vendor fixed versions.
  • Review kernel logs for UBSAN or Kyber-related out-of-bounds reports.
Prepared
Confidence
medium
Sources
7

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-2021-46984 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
6Source 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
LinuxLinuxa6088845c2bf754d6cb2572b484180680b037804, a6088845c2bf754d6cb2572b484180680b037804, a6088845c2bf754d6cb2572b484180680b037804, a6088845c2bf754d6cb2572b484180680b037804, a6088845c2bf754d6cb2572b484180680b037804unaffected
LinuxLinux4.18, 0, 5.4.120, 5.10.38, 5.11.22, 5.12.5, 5.13affected
Weakness

CWE details

No CWE listed

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