CVE-2026-46251 btrfs: fix block_group_tree dirty_list corruption
In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix block_group_tree dirty_list corruption

When the incompat flag EXTENT_TREE_V2 is set, we unconditionally add the
block group tree to the switch_commits list before calling
switch_commit_roots, as we do for the tree root and the chunk root.
However, the block group tree uses normal root dirty tracking and in any
transaction that does an allocation and dirties a block group, the block
group root will already be linked to a list by the dirty_list field and
this use of list_add_tail() is invalid and corrupts the prev/next
members of block_group_root->dirty_list.

This is apparent on a subsequent list_del on the prev if we enable
CONFIG_DEBUG_LIST:

  [32.1571] ------------[ cut here ]------------
  [32.1572] list_del corruption. next->prev should beffff958890202538, but was ffff9588992bd538. (next=ffff958890201538)
  [32.1575] WARNING: lib/list_debug.c:65 at 0x0, CPU#3: sync/607
  [32.1583] CPU: 3 UID: 0 PID: 607 Comm: sync Not tainted 6.18.0 #24PREEMPT(none)
  [32.1585] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS1.17.0-4.fc41 04/01/2014
  [32.1587] RIP: 0010:__list_del_entry_valid_or_report+0x108/0x120
  [32.1593] RSP: 0018:ffffaa288287fdd0 EFLAGS: 00010202
  [32.1594] RAX: 0000000000000001 RBX: ffff95889326e800 RCX:ffff958890201538
  [32.1596] RDX: ffff9588992bd538 RSI: ffff958890202538 RDI:ffffffff82a41e00
  [32.1597] RBP: ffff958890202538 R08: ffffffff828fc1e8 R09:00000000ffffefff
  [32.1599] R10: ffffffff8288c200 R11: ffffffff828e4200 R12:ffff958890201538
  [32.1601] R13: ffff95889326e958 R14: ffff958895c24000 R15:ffff958890202538
  [32.1603] FS:  00007f0c28eb5740(0000) GS:ffff958af2bd2000(0000)knlGS:0000000000000000
  [32.1605] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [32.1607] CR2: 00007f0c28e8a3cc CR3: 0000000109942005 CR4:0000000000370ef0
  [32.1609] Call Trace:
  [32.1610]  <TASK>
  [32.1611]  switch_commit_roots+0x82/0x1d0 [btrfs]
  [32.1615]  btrfs_commit_transaction+0x968/0x1550 [btrfs]
  [32.1618]  ? btrfs_attach_transaction_barrier+0x23/0x60 [btrfs]
  [32.1621]  __iterate_supers+0xe8/0x190
  [32.1622]  ? __pfx_sync_fs_one_sb+0x10/0x10
  [32.1623]  ksys_sync+0x63/0xb0
  [32.1624]  __do_sys_sync+0xe/0x20
  [32.1625]  do_syscall_64+0x73/0x450
  [32.1626]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
  [32.1627] RIP: 0033:0x7f0c28d05d2b
  [32.1632] RSP: 002b:00007ffc9d988048 EFLAGS: 00000246 ORIG_RAX:00000000000000a2
  [32.1634] RAX: ffffffffffffffda RBX: 00007ffc9d988228 RCX:00007f0c28d05d2b
  [32.1636] RDX: 00007f0c28e02301 RSI: 00007ffc9d989b21 RDI:00007f0c28dba90d
  [32.1637] RBP: 0000000000000001 R08: 0000000000000001 R09:0000000000000000
  [32.1639] R10: 0000000000000000 R11: 0000000000000246 R12:000055b96572cb80
  [32.1641] R13: 000055b96572b19f R14: 00007f0c28dfa434 R15:000055b96572b034
  [32.1643]  </TASK>
  [32.1644] irq event stamp: 0
  [32.1644] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
  [32.1646] hardirqs last disabled at (0): [<ffffffff81298817>]copy_process+0xb37/0x2260
  [32.1648] softirqs last  enabled at (0): [<ffffffff81298817>]copy_process+0xb37/0x2260
  [32.1650] softirqs last disabled at (0): [<0000000000000000>] 0x0
  [32.1652] ---[ end trace 0000000000000000 ]---

Furthermore, this list corruption eventually (when we happen to add a
new block group) results in getting the switch_commits and
dirty_cowonly_roots lists mixed up and attempting to call update_root
on the tree root which can't be found in the tree root, resulting in a
transaction abort:

  [87.8269] BTRFS critical (device nvme1n1): unable to find root key (1 0 0) in tree 1
  [87.8272] ------------[ cut here ]------------
  [87.8274] BTRFS: Transaction aborted (error -117)
  [87.8275] WARNING: fs/btrfs/root-tree.c:153 at 0x0, CPU#4: sync/703
  [87.8285] CPU: 4 UID: 0 PID: 703 Comm: sync Not tainted 6.18.0 #25 PREEMPT(none)
  [87.8287] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-4.fc41 0
---truncated---
CVSS v3.1 score: 8.4