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CVE-2026-46159 Linux CVE debrief

A Time-of-Check to Time-of-Use (TOCTOU) race condition in the Linux kernel's Btrfs filesystem driver can lead to information disclosure. The vulnerability exists in `btrfs_ioctl_space_info()`, which performs two passes over block group RAID type lists: first to count entries and allocate a buffer, then to fill that buffer. The `groups_sem` rwlock is released between these passes, allowing concurrent block group removal to reduce the entry count. When the second pass fills fewer entries than the first pass counted, `copy_to_user()` copies the full allocated buffer size—including trailing uninitialized kernel heap memory—to userspace. The fix copies only the actually-filled entry count and switches to `kzalloc` to prevent heap data leakage from any future size mismatches.

Vendor
Linux
Product
Unknown
CVSS
MEDIUM 4.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-28
Original CVE updated
2026-06-19
Advisory published
2026-05-28
Advisory updated
2026-06-19

Who should care

Linux system administrators running Btrfs filesystems, kernel security teams, and organizations with strict kernel memory isolation requirements

Technical summary

The `btrfs_ioctl_space_info()` function in the Linux kernel Btrfs driver contains a TOCTOU vulnerability where the `groups_sem` lock is dropped between counting block group entries and filling the user buffer. Concurrent block group removal can reduce the actual entry count below the allocated buffer size, causing `copy_to_user()` to copy uninitialized kernel heap memory to userspace. The remediation copies only filled entries and uses `kzalloc` for defense-in-depth.

Defensive priority

medium

Recommended defensive actions

  • Apply kernel updates containing the referenced stable commits when available from your Linux distribution
  • Monitor vendor security advisories for patched kernel packages
  • Restrict untrusted user access to Btrfs ioctl interfaces until patching is complete
  • Review systems with Btrfs filesystems for unauthorized access patterns

Evidence notes

CVE description confirms TOCTOU race between allocation-size counting pass and data-filling pass with `groups_sem` released between them. Kernel commit references indicate stable backports. No CVSS score or severity assigned by NVD at time of disclosure.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-46159 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-46159

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

  • CVE-2026-46159 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-46159

    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/4fdc6ee0802121d9cd96b8d085e589f51e5a4ec3

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

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/5d12e0ab009ade48c1bff9324fd9bea2c773d088

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

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/973e57c726c1f8e77259d1c8e519519f1e9aea77

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

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/d09d67d5de577cedae3de9497dff217e0ac8b641

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

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/f5ee467b56764964027c361641f64953fc0f8f9a

    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.