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CVE Record

CVE-2025-37856: btrfs: harden block_group::bg_list against list_del() races

In the Linux kernel, the following vulnerability has been resolved: btrfs: harden block_group::bg_list against list_del() races As far as I can tell, these calls of list_del_init() on bg_list cannot run concurrently with btrfs_mark_bg_unused() or btrfs_mark_bg_to_reclaim(), as they are in transaction error paths and situations where the block group is readonly. However, if there is any chance at all of racing with mark_bg_unused(), or a different future user of bg_list, better to be safe than sorry. Otherwise we risk the following interleaving (bg_list refcount in parens) T1 (some random op) T2 (btrfs_mark_bg_unused) !list_empty(&bg->bg_list); (1) list_del_init(&bg->bg_list); (1) list_move_tail (1) btrfs_put_block_group (0) btrfs_delete_unused_bgs bg = list_first_entry list_del_init(&bg->bg_list); btrfs_put_block_group(bg); (-1) Ultimately, this results in a broken ref count that hits zero one deref early and the real final deref underflows the refcount, resulting in a WARNING.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A race in Linux Btrfs block-group bookkeeping can corrupt a reference count, potentially releasing an object too early and later underflowing its count. The supplied record rates potential confidentiality, integrity, and availability impact as high, but documents a kernel warning rather than a demonstrated compromise. Exposure requires an affected kernel, Btrfs use, and local low-privileged access.

Executive priority

Schedule prompt remediation for Btrfs systems that permit local untrusted access, especially shared servers and workload hosts. Internet exposure alone does not create the stated attack path. Validate vendor backports before declaring systems vulnerable because upstream version matching can misclassify distribution kernels. Systems not using Btrfs are lower priority for this specific flaw.

Technical view

Concurrent manipulation of block_group::bg_list may interleave list deletion, movement, and reference release. This can reduce the block-group reference count to zero one dereference early, followed by an underflow and kernel WARNING. The fix hardens list_del_init handling against possible current or future races. CVSS 3.1 is 7.8: local, low complexity, low privileges, and no user interaction.

Likely exposure

Prioritize Linux systems running affected kernel branches with Btrfs filesystems mounted or otherwise exercised. The supplied affected-version data includes 6.5, 6.12.24, 6.13.12, 6.14.3, and 6.15, but contains ambiguous entries and may not reflect distribution backports. This is not described as remotely exploitable; the CVSS vector requires local low-privileged access.

Exploitation context

The supplied sources do not establish active exploitation, and the CVE is not identified as KEV. They describe a possible race and resulting reference-count warning, without a public proof of exploitation or confirmed path from the warning to code execution. Treat the CVSS impact ratings as potential consequences, not evidence that each consequence has been demonstrated.

Researcher notes

The record assigns no CWE. Its narrative is explicitly defensive: the author could not confirm that the identified calls currently race, but hardened them against any possibility and future users. The concrete documented failure is premature reference release, later underflow, and a WARNING. Evidence is insufficient to claim reliable privilege escalation, memory corruption exploitation, or remote reachability.

Mitigation direction

  • Inventory Linux systems using Btrfs and identify their exact kernel packages and distribution backports.
  • Apply the vendor-supported kernel update containing the applicable upstream stable fix.
  • Check distribution security guidance because package versions may not match upstream affected-version numbering.
  • Prioritize shared or multi-user Btrfs systems where untrusted users have local accounts or workload access.

Validation and detection

  • Confirm whether each system mounts or otherwise uses a Btrfs filesystem.
  • Compare installed kernel packages against vendor advisories and their documented backported fixes.
  • Verify the updated kernel includes an applicable referenced stable commit or vendor-equivalent backport.
  • After updating and rebooting, confirm the running kernel is the remediated package.
  • Review kernel logs for Btrfs reference-count warnings while recognizing absence of warnings does not prove safety.
Prepared
Confidence
medium
Sources
6

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-2025-37856 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

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.

1CVSS vectors
3Timeline events
0ADP providers
5Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2025-37856Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

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.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxa9f189716cf15913c453299d72f69c51a9b0f86b, a9f189716cf15913c453299d72f69c51a9b0f86b, a9f189716cf15913c453299d72f69c51a9b0f86b, a9f189716cf15913c453299d72f69c51a9b0f86b, edf3b5aadb2515c808200b904baa5b70a727f0ac, 01eca70ef8cf499d0cb6d1bbd691558e7792cf17, 5d19abcffd8404078dfa7d7118cec357b5e7bc58, 5.15.128, 6.1.47, 6.4.12unaffected
LinuxLinux6.5, 0, 6.12.24, 6.13.12, 6.14.3, 6.15affected
Weakness

CWE details

No CWE listed

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