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:findfreeextentupdateloop fs/btrfs/extent-tree.c:4208 [inline] RIP: 0010:findfreeextent+0x52f0/0x5d20 fs/btrfs/extent-tree.c:4611 Call Trace: <TASK> btrfsreserveextent+0x2cd/0x790 fs/btrfs/extent-tree.c:4705 btrfsalloctreeblock+0x1e1/0x10e0 fs/btrfs/extent-tree.c:5157 btrfsforcecowblock+0x578/0x2410 fs/btrfs/ctree.c:517 btrfscowblock+0x3c4/0xa80 fs/btrfs/ctree.c:708 btrfssearchslot+0xcad/0x2b50 fs/btrfs/ctree.c:2130 btrfstruncateinodeitems+0x45d/0x2350 fs/btrfs/inode-item.c:499 btrfsevictinode+0x923/0xe70 fs/btrfs/inode.c:5628 evict+0x5f4/0xae0 fs/inode.c:837 dentrykill+0x209/0x660 fs/dcache.c:670 finishdput+0xc9/0x480 fs/dcache.c:879 shrinkdcacheforumount+0xa0/0x170 fs/dcache.c:1661 genericshutdownsuper+0x67/0x2c0 fs/super.c:621 killanonsuper+0x3b/0x70 fs/super.c:1289 btrfskillsuper+0x41/0x50 fs/btrfs/super.c:2127 deactivatelockedsuper+0xbc/0x130 fs/super.c:474 cleanupmnt+0x425/0x4c0 fs/namespace.c:1318 taskworkrun+0x1d4/0x260 kernel/taskwork.c:233 exittaskwork include/linux/taskwork.h:40 [inline] doexit+0x694/0x22f0 kernel/exit.c:971 dogroupexit+0x21c/0x2d0 kernel/exit.c:1112 dosysexitgroup kernel/exit.c:1123 [inline] sesysexitgroup kernel/exit.c:1121 [inline] x64sysexitgroup+0x3f/0x40 kernel/exit.c:1121 x64syscall+0x2210/0x2210 arch/x86/include/generated/asm/syscalls64.h:232 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xe8/0xf80 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x44f639 Code: Unable to access opcode bytes at 0x44f60f. RSP: 002b:00007ffc15c4e088 EFLAGS: 00000246 ORIGRAX: 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.