btrfs: harden block_group::bg_list against list_del() races
CVE-2025-37856 · published 9 May 2025
How serious is it
- Severity: high, CVSS score 7.8 out of 10.
- No exploit prediction (EPSS) score yet.
- Not on the CISA list of exploited vulnerabilities.
- No public exploit in Exploit-DB.
Is your app affected by CVE-2025-37856?
Paste this prompt into Claude Code, Cursor or any AI coding tool: it runs a free Shippable scan of your project and fixes what it finds. Built with Lovable, Bolt or v0? Scan your live app. Or upload your package file.
Who is affected
- Linux Linux: a9f189716cf15913c453299d72f69c51a9b0f86b to < 9039afe6240ae01d6caf2fa731590ac74fb8a69b, a9f189716cf15913c453299d72f69c51a9b0f86b to < bf089c4d1141b27332c092b1dcca5022c415a3b6, a9f189716cf15913c453299d72f69c51a9b0f86b to < 909e60fb469d4101c6b08cf6e622efb062bb24a1, a9f189716cf15913c453299d72f69c51a9b0f86b to < 185fd73e5ac06027c4be9a129e59193f6a3ef202
- Linux Linux: 6.5
How to check
- Find out whether you run the affected product and which version, then compare it with the versions above.
How to fix
Copies a prompt for Claude Code, Cursor or Codex that asks it to check whether you run this product.
- Apply the vendor's update or mitigation from the references below. If there is none yet, limit exposure of the affected service until there is.
- If a vulnerable version was live, check your logs and rotate the secrets that server could read.
The advisory
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.
Check your whole app
Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.
References
- git.kernel.org/stable/c/9039afe6240ae01d6caf2fa731590ac74fb8a69b
- git.kernel.org/stable/c/bf089c4d1141b27332c092b1dcca5022c415a3b6
- git.kernel.org/stable/c/909e60fb469d4101c6b08cf6e622efb062bb24a1
- git.kernel.org/stable/c/185fd73e5ac06027c4be9a129e59193f6a3ef202
- git.kernel.org/stable/c/7511e29cf1355b2c47d0effb39e463119913e2f6
Merged from CVE List. Last updated 25 Oct 2026. All vulnerabilities · RSS · JSON