CVE-2025-22115: 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---