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

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-05-09 · updated 2026-10-30

Description

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.

Affected products

Linux kernel: before 6.12.24; from 6.13 before 6.13.12; from 6.14 before 6.14.3 — fixed in 6.12.24, 6.13.12, 6.14.3
Linux: from 6.1.47 before 6.1.191; from 5.15.128 before 5.16; from 6.4.12 before 6.5 — fixed in 6.1.191, 5.16, 6.5

How to fix

Upgrade to a fixed version: Linux kernel 6.12.24, 6.13.12, 6.14.3; Linux 6.1.191, 5.16, 6.5. Check the vendor advisory for mitigations.

References

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