{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-22115",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-22115"
  ],
  "published": "2025-04-16T14:14:59Z",
  "summary": "btrfs: fix block group refcount race in btrfs_create_pending_block_groups()",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbtrfs: fix block group refcount race in btrfs_create_pending_block_groups()\n\nBlock group creation is done in two phases, which results in a slightly\nunintuitive property: a block group can be allocated/deallocated from\nafter btrfs_make_block_group() adds it to the space_info with\nbtrfs_add_bg_to_space_info(), but before creation is completely completed\nin btrfs_create_pending_block_groups(). As a result, it is possible for a\nblock group to go unused and have 'btrfs_mark_bg_unused' called on it\nconcurrently with 'btrfs_create_pending_block_groups'. This causes a\nnumber of issues, which were fixed with the block group flag\n'BLOCK_GROUP_FLAG_NEW'.\n\nHowever, this fix is not quite complete. Since it does not use the\nunused_bg_lock, it is possible for the following race to occur:\n\nbtrfs_create_pending_block_groups            btrfs_mark_bg_unused\n                                           if list_empty // false\n        list_del_init\n        clear_bit\n                                           else if (test_bit) // true\n                                                list_move_tail\n\nAnd we get into the exact same broken ref count and invalid new_bgs\nstate for transaction cleanup that BLOCK_GROUP_FLAG_NEW was designed to\nprevent.\n\nThe broken refcount aspect will result in a warning like:\n\n  [1272.943527] refcount_t: underflow; use-after-free.\n  [1272.943967] WARNING: CPU: 1 PID: 61 at lib/refcount.c:28 refcount_warn_saturate+0xba/0x110\n  [1272.944731] Modules linked in: btrfs virtio_net xor zstd_compress raid6_pq null_blk [last unloaded: btrfs]\n  [1272.945550] CPU: 1 UID: 0 PID: 61 Comm: kworker/u32:1 Kdump: loaded Tainted: G        W          6.14.0-rc5+ #108\n  [1272.946368] Tainted: [W]=WARN\n  [1272.946585] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014\n  [1272.947273] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs]\n  [1272.947788] RIP: 0010:refcount_warn_saturate+0xba/0x110\n  [1272.949532] RSP: 0018:ffffbf1200247df0 EFLAGS: 00010282\n  [1272.949901] RAX: 0000000000000000 RBX: ffffa14b00e3f800 RCX: 0000000000000000\n  [1272.950437] RDX: 0000000000000000 RSI: ffffbf1200247c78 RDI: 00000000ffffdfff\n  [1272.950986] RBP: ffffa14b00dc2860 R08: 00000000ffffdfff R09: ffffffff90526268\n  [1272.951512] R10: ffffffff904762c0 R11: 0000000063666572 R12: ffffa14b00dc28c0\n  [1272.952024] R13: 0000000000000000 R14: ffffa14b00dc2868 R15: 000001285dcd12c0\n  [1272.952850] FS:  0000000000000000(0000) GS:ffffa14d33c40000(0000) knlGS:0000000000000000\n  [1272.953458] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033\n  [1272.953931] CR2: 00007f838cbda000 CR3: 000000010104e000 CR4: 00000000000006f0\n  [1272.954474] Call Trace:\n  [1272.954655]  <TASK>\n  [1272.954812]  ? refcount_warn_saturate+0xba/0x110\n  [1272.955173]  ? __warn.cold+0x93/0xd7\n  [1272.955487]  ? refcount_warn_saturate+0xba/0x110\n  [1272.955816]  ? report_bug+0xe7/0x120\n  [1272.956103]  ? handle_bug+0x53/0x90\n  [1272.956424]  ? exc_invalid_op+0x13/0x60\n  [1272.956700]  ? asm_exc_invalid_op+0x16/0x20\n  [1272.957011]  ? refcount_warn_saturate+0xba/0x110\n  [1272.957399]  btrfs_discard_cancel_work.cold+0x26/0x2b [btrfs]\n  [1272.957853]  btrfs_put_block_group.cold+0x5d/0x8e [btrfs]\n  [1272.958289]  btrfs_discard_workfn+0x194/0x380 [btrfs]\n  [1272.958729]  process_one_work+0x130/0x290\n  [1272.959026]  worker_thread+0x2ea/0x420\n  [1272.959335]  ? __pfx_worker_thread+0x10/0x10\n  [1272.959644]  kthread+0xd7/0x1c0\n  [1272.959872]  ? __pfx_kthread+0x10/0x10\n  [1272.960172]  ret_from_fork+0x30/0x50\n  [1272.960474]  ? __pfx_kthread+0x10/0x10\n  [1272.960745]  ret_from_fork_asm+0x1a/0x30\n  [1272.961035]  </TASK>\n  [1272.961238] ---[ end trace 0000000000000000 ]---\n\nThough we have seen them in the async discard workfn as well. It is\nmost likely to happen after a relocation finishes which cancels discard,\ntears down the block group, etc.\n\nFix this fully by taking the lock arou\n---truncated---",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-22115"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/ee56da95f8962b86fec4ef93f866e64c8d025a58"
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    "upstream_modified": "2026-08-05T12:19:11Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The vulnerability is reached through ordinary filesystem write/delete syscalls against a mounted btrfs filesystem, requiring a local account on the system. There is no network-facing entry point into btrfs chunk/block-group allocation.\nAC:L - The attacker drives both sides of the race \u2014 one thread forces new chunk/block-group creation via writes, another frees the last extents in that group so btrfs_mark_bg_unused() fires \u2014 and can retry the interleaving indefinitely with high thread concurrency. No special kernel config or mount option is required, since the default (non-discard=async) btrfs_update_block_group() path calls btrfs_mark_bg_unused() directly.\nPR:L - Only write access to any btrfs filesystem is needed, which an unprivileged local user has on the common case of a btrfs root/home/tmp; no capability, CAP_SYS_ADMIN, or ioctl gate exists on this path. No privileged operation such as balance is required \u2014 the generic extent-free path suffices.\nUI:N - The filesystem is already mounted in the normal case and the attacker triggers everything itself with plain write/unlink activity. No victim action of any kind is needed.\nS:U - The corrupted object and all its consequences are inside the kernel's own security authority; there is no crossing of a VM, IOMMU, or sandbox boundary. This is a standard kernel-local memory-safety issue.\nC:H - The premature refcount drop frees struct btrfs_block_group while it remains reachable from the block-group rbtree, space_info lists, and discard/dirty lists, so kernel workers keep reading attacker-reclaimable slab memory. Since the object comes from a generic kmalloc cache, the attacker can spray it and turn the stale dereferences into an arbitrary kernel-memory read primitive.\nI:H - The freed-but-still-linked block group is later written through and unlinked (list_del/list_move on bg_list, discard_list, dirty_list), giving a classic unlink write-what-where once the slot is reclaimed with attacker-controlled data. Its embedded pointers (free_space_ctl, physical_map, inode, caching_ctl) are dereferenced afterwards, making control-flow hijack and privilege escalation feasible.\nA:H - Even unexploited, the bug produces refcount underflow WARNs, corrupted new_bgs/unused_bgs list state, and use-after-free of a structure central to allocation, which reliably leads to oops/panic and an unusable filesystem. The reported crash occurs in a btrfs kernel worker thread, taking down the whole system rather than just the attacking task."
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}