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CVE-2026-46066 Linux CVE debrief

A kernel panic vulnerability exists in the Linux kernel's CephFS client when handling encrypted file writeback. The issue stems from an off-by-one error in `ceph_wbc->num_ops` accounting when bounce buffer allocation fails during multi-folio encrypted writeback operations. Specifically, when `move_dirty_folio_in_page_array()` fails due to memory pressure on a non-first folio in a batch, `ceph_process_folio_batch()` increments `num_ops` for a new extent but then fails to add the discontiguous folio to the array. This causes `ceph_submit_write()` to trigger a `BUG_ON()` panic when the operation index does not match the expected number of operations. The vulnerability was introduced by commit cac190c7674f and unmasked in kernel versions 6.18.16, 6.19.6, and 7.0-rc1 when a separate bug fix enabled multi-folio encrypted writeback. The underlying defect has existed since 2022. Exploitation requires fscrypt-enabled CephFS with specific write patterns (4KiB written/4KiB skipped) under memory pressure conditions. The fix decrements `num_ops` when a new extent is started but the folio fails to be added to the batch.

Vendor
Linux
Product
Unknown
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-27
Original CVE updated
2026-06-24
Advisory published
2026-05-27
Advisory updated
2026-06-24

Who should care

Organizations running Linux kernels 6.18.16, 6.19.6, or 7.0-rc1 with CephFS and fscrypt encryption enabled; Ceph storage administrators; Linux distribution security teams; cloud infrastructure providers offering Ceph-based storage with encryption; security operations teams monitoring for kernel stability issues

Technical summary

The vulnerability is an off-by-one accounting error in the CephFS client's writeback path. When processing encrypted folio batches, `ceph_process_folio_batch()` increments `ceph_wbc->num_ops` when encountering a discontiguous dirty folio, indicating a new write extent operation will be needed. If `move_dirty_folio_in_page_array()` subsequently fails due to bounce buffer allocation failure (possible under memory pressure), the folio is redirtied for future retry but is not added to the current batch. However, `num_ops` is not decremented, leaving it one higher than the actual number of contiguous ranges in the batch. When `ceph_submit_write()` executes, its `BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops)` assertion fails because the operation index tracking does not match the inflated `num_ops` value, causing a kernel panic. The fix adds a decrement of `num_ops` when this specific failure condition occurs on a newly-started extent.

Defensive priority

high

Recommended defensive actions

  • Apply kernel patches from stable tree commits 6200f41d6fcf, a0d9555bf9ea, and ba12c1e57889 when available for your distribution
  • Prioritize patching systems running kernel 6.18.16, 6.19.6, or 7.0-rc1 with CephFS fscrypt encryption enabled
  • Monitor for kernel panic traces containing 'BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops)' as indicator of exploitation attempt
  • Consider memory pressure monitoring on CephFS clients with encrypted workloads to detect conditions favorable to trigger
  • Review CephFS writeback patterns and consider workload adjustments if patching is delayed
  • Validate backup and recovery procedures for CephFS encrypted data given potential for system instability

Evidence notes

The CVE description provides detailed technical analysis including the specific BUG_ON trigger, affected code paths (ceph_process_folio_batch, ceph_submit_write, move_dirty_folio_in_page_array), and reproduction conditions (fscrypt-enabled CephFS, 4KiB write/skip pattern, memory pressure). Three kernel.org stable tree commits are referenced as fixes. The vulnerability status is 'Awaiting Analysis' per NVD.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-46066 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-46066

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-46066 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-46066

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/6200f41d6fcf2ac7e24866431e381cbc914560e4

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/a0d9555bf9eaeba34fe6b6bb86f442fe08ba3842

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/ba12c1e578890f6337a415b7dedf476c6d455105

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.