Description

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_wa...

Classification

CVE ID: CVE-2025-22115

Affected Products

Vendor: Linux

Product: Linux, Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.02% (probability of being exploited)

EPSS Percentile: 3.27% (scored less or equal to compared to others)

EPSS Date: 2025-04-18 (when was this score calculated)

References

https://nvd.nist.gov/vuln/detail/CVE-2025-22115
https://git.kernel.org/stable/c/9d383a6fc59271aaaf07a33b23b2eac5b9268b7a
https://git.kernel.org/stable/c/2d8e5168d48a91e7a802d3003e72afb4304bebfa

Timeline