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CVE Record

CVE-2025-22115: btrfs: fix block group refcount race in btrfs_create_pending_block_groups()

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix block group refcount race in btrfs_create_pending_block_groups() Block group creation is done in two phases, which results in a slightly unintuitive property: a block group can be allocated/deallocated from after btrfs_make_block_group() adds it to the space_info with btrfs_add_bg_to_space_info(), but before creation is completely completed in btrfs_create_pending_block_groups(). As a result, it is possible for a block group to go unused and have 'btrfs_mark_bg_unused' called on it concurrently with 'btrfs_create_pending_block_groups'. This causes a number of issues, which were fixed with the block group flag 'BLOCK_GROUP_FLAG_NEW'. However, this fix is not quite complete. Since it does not use the unused_bg_lock, it is possible for the following race to occur: btrfs_create_pending_block_groups btrfs_mark_bg_unused if list_empty // false list_del_init clear_bit else if (test_bit) // true list_move_tail And we get into the exact same broken ref count and invalid new_bgs state for transaction cleanup that BLOCK_GROUP_FLAG_NEW was designed to prevent. The broken refcount aspect will result in a warning like: [1272.943527] refcount_t: underflow; use-after-free. [1272.943967] WARNING: CPU: 1 PID: 61 at lib/refcount.c:28 refcount_warn_saturate+0xba/0x110 [1272.944731] Modules linked in: btrfs virtio_net xor zstd_compress raid6_pq null_blk [last unloaded: btrfs] [1272.945550] CPU: 1 UID: 0 PID: 61 Comm: kworker/u32:1 Kdump: loaded Tainted: G W 6.14.0-rc5+ #108 [1272.946368] Tainted: [W]=WARN [1272.946585] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014 [1272.947273] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs] [1272.947788] RIP: 0010:refcount_warn_saturate+0xba/0x110 [1272.949532] RSP: 0018:ffffbf1200247df0 EFLAGS: 00010282 [1272.949901] RAX: 0000000000000000 RBX: ffffa14b00e3f800 RCX: 0000000000000000 [1272.950437] RDX: 0000000000000000 RSI: ffffbf1200247c78 RDI: 00000000ffffdfff [1272.950986] RBP: ffffa14b00dc2860 R08: 00000000ffffdfff R09: ffffffff90526268 [1272.951512] R10: ffffffff904762c0 R11: 0000000063666572 R12: ffffa14b00dc28c0 [1272.952024] R13: 0000000000000000 R14: ffffa14b00dc2868 R15: 000001285dcd12c0 [1272.952850] FS: 0000000000000000(0000) GS:ffffa14d33c40000(0000) knlGS:0000000000000000 [1272.953458] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [1272.953931] CR2: 00007f838cbda000 CR3: 000000010104e000 CR4: 00000000000006f0 [1272.954474] Call Trace: [1272.954655] <TASK> [1272.954812] ? refcount_warn_saturate+0xba/0x110 [1272.955173] ? __warn.cold+0x93/0xd7 [1272.955487] ? refcount_warn_saturate+0xba/0x110 [1272.955816] ? report_bug+0xe7/0x120 [1272.956103] ? handle_bug+0x53/0x90 [1272.956424] ? exc_invalid_op+0x13/0x60 [1272.956700] ? asm_exc_invalid_op+0x16/0x20 [1272.957011] ? refcount_warn_saturate+0xba/0x110 [1272.957399] btrfs_discard_cancel_work.cold+0x26/0x2b [btrfs] [1272.957853] btrfs_put_block_group.cold+0x5d/0x8e [btrfs] [1272.958289] btrfs_discard_workfn+0x194/0x380 [btrfs] [1272.958729] process_one_work+0x130/0x290 [1272.959026] worker_thread+0x2ea/0x420 [1272.959335] ? __pfx_worker_thread+0x10/0x10 [1272.959644] kthread+0xd7/0x1c0 [1272.959872] ? __pfx_kthread+0x10/0x10 [1272.960172] ret_from_fork+0x30/0x50 [1272.960474] ? __pfx_kthread+0x10/0x10 [1272.960745] ret_from_fork_asm+0x1a/0x30 [1272.961035] </TASK> [1272.961238] ---[ end trace 0000000000000000 ]--- Though we have seen them in the async discard workfn as well. It is most likely to happen after a relocation finishes which cancels discard, tears down the block group, etc. Fix this fully by taking the lock arou ---truncated---

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2025-22115 is a Linux kernel Btrfs bug where a race condition can mishandle internal block group reference counts. In practical terms, affected systems using Btrfs may hit kernel warnings and instability during storage operations. The source bundle does not show internet-facing exploitation or a CVSS score.

Executive priority

Treat this as a targeted kernel maintenance priority for Btrfs-dependent infrastructure, not a broad emergency across all Linux systems. Prioritize environments where Btrfs stability affects production availability or data operations.

Technical view

The flaw is in btrfs_create_pending_block_groups(), where btrfs_mark_bg_unused can race with pending block group creation. Missing unused_bg_lock coverage can leave broken refcounts and invalid new_bgs transaction cleanup state, producing refcount underflow and possible use-after-free warnings.

Likely exposure

Exposure is most relevant to Linux hosts using Btrfs, especially systems performing relocation, discard, or block group lifecycle activity. Linux systems not using Btrfs have much lower practical exposure. Exact affected version boundaries should be validated against kernel stable advisories because the bundled version data is incomplete and partly commit-based.

Exploitation context

The bundle marks KEV as false and provides no evidence of active exploitation. The described trigger is a kernel race in Btrfs block group handling, not a remote protocol issue. Sources show reliability and memory-safety symptoms but do not provide exploitability, attacker prerequisites, or impact scoring.

Researcher notes

The evidence supports a Btrfs race involving block group creation, unused block group handling, and transaction cleanup state. The bundle does not include CVSS, CWE, exploit proof, or confirmed attacker model, so impact assessment should remain conservative until vendor advisories add detail.

Mitigation direction

  • Review kernel vendor guidance for CVE-2025-22115 and Btrfs fixes.
  • Upgrade to a Linux kernel containing the referenced stable fixes.
  • Prioritize Btrfs storage servers, virtualization hosts, and systems using discard or relocation workflows.
  • If immediate upgrade is unavailable, reduce nonessential Btrfs maintenance churn where operationally safe.
  • Track distribution advisories for backported fixes rather than relying only on upstream version numbers.

Validation and detection

  • Inventory Linux hosts using Btrfs filesystems.
  • Compare running kernel builds with distribution advisories for CVE-2025-22115.
  • Check kernel logs for refcount underflow or Btrfs discard work warnings.
  • Confirm patched kernels include one of the referenced stable commits or vendor backports.
  • Regression-test Btrfs workloads after upgrade, especially relocation and discard-heavy operations.
Prepared
Confidence
medium
Sources
5

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

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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
0ADP providers
4Source links

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

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Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux0657b20c5a76c938612f8409735a8830d257866e, 0657b20c5a76c938612f8409735a8830d257866e, 0657b20c5a76c938612f8409735a8830d257866e, 6297644db23f77c02ae7961cc542d162629ae2c4, 7569c4294ba6ff9f194635b14876198f8a687c4a, 6.1.47, 6.4.12unaffected
LinuxLinux6.5, 0, 6.12.40, 6.14.2, 6.15affected
Weakness

CWE details

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