CVE-2026-23214: btrfs: reject new transactions if the fs is fully read-only
In the Linux kernel, the following vulnerability has been resolved:

btrfs: reject new transactions if the fs is fully read-only

[BUG]
There is a bug report where a heavily fuzzed fs is mounted with all
rescue mount options, which leads to the following warnings during
unmount:

  BTRFS: Transaction aborted (error -22)
  Modules linked in:
  CPU: 0 UID: 0 PID: 9758 Comm: repro.out Not tainted
  6.19.0-rc5-00002-gb71e635feefc #7 PREEMPT(full)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  RIP: 0010:find_free_extent_update_loop fs/btrfs/extent-tree.c:4208 [inline]
  RIP: 0010:find_free_extent+0x52f0/0x5d20 fs/btrfs/extent-tree.c:4611
  Call Trace:
   <TASK>
   btrfs_reserve_extent+0x2cd/0x790 fs/btrfs/extent-tree.c:4705
   btrfs_alloc_tree_block+0x1e1/0x10e0 fs/btrfs/extent-tree.c:5157
   btrfs_force_cow_block+0x578/0x2410 fs/btrfs/ctree.c:517
   btrfs_cow_block+0x3c4/0xa80 fs/btrfs/ctree.c:708
   btrfs_search_slot+0xcad/0x2b50 fs/btrfs/ctree.c:2130
   btrfs_truncate_inode_items+0x45d/0x2350 fs/btrfs/inode-item.c:499
   btrfs_evict_inode+0x923/0xe70 fs/btrfs/inode.c:5628
   evict+0x5f4/0xae0 fs/inode.c:837
   __dentry_kill+0x209/0x660 fs/dcache.c:670
   finish_dput+0xc9/0x480 fs/dcache.c:879
   shrink_dcache_for_umount+0xa0/0x170 fs/dcache.c:1661
   generic_shutdown_super+0x67/0x2c0 fs/super.c:621
   kill_anon_super+0x3b/0x70 fs/super.c:1289
   btrfs_kill_super+0x41/0x50 fs/btrfs/super.c:2127
   deactivate_locked_super+0xbc/0x130 fs/super.c:474
   cleanup_mnt+0x425/0x4c0 fs/namespace.c:1318
   task_work_run+0x1d4/0x260 kernel/task_work.c:233
   exit_task_work include/linux/task_work.h:40 [inline]
   do_exit+0x694/0x22f0 kernel/exit.c:971
   do_group_exit+0x21c/0x2d0 kernel/exit.c:1112
   __do_sys_exit_group kernel/exit.c:1123 [inline]
   __se_sys_exit_group kernel/exit.c:1121 [inline]
   __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1121
   x64_sys_call+0x2210/0x2210 arch/x86/include/generated/asm/syscalls_64.h:232
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0xe8/0xf80 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  RIP: 0033:0x44f639
  Code: Unable to access opcode bytes at 0x44f60f.
  RSP: 002b:00007ffc15c4e088 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
  RAX: ffffffffffffffda RBX: 00000000004c32f0 RCX: 000000000044f639
  RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000001
  RBP: 0000000000000001 R08: ffffffffffffffc0 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004c32f0
  R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001
   </TASK>

Since rescue mount options will mark the full fs read-only, there should
be no new transaction triggered.

But during unmount we will evict all inodes, which can trigger a new
transaction, and triggers warnings on a heavily corrupted fs.

[CAUSE]
Btrfs allows new transaction even on a read-only fs, this is to allow
log replay happen even on read-only mounts, just like what ext4/xfs do.

However with rescue mount options, the fs is fully read-only and cannot
be remounted read-write, thus in that case we should also reject any new
transactions.

[FIX]
If we find the fs has rescue mount options, we should treat the fs as
error, so that no new transaction can be started.
CVSS v3.1: 5.5