f2fs: fix to don't set SB_RDONLY in f2fs_handle_critical_error()

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix to don't set SB_RDONLY in f2fs_handle_critical_error()

syzbot reports a f2fs bug as below:

------------[ cut here ]------------

WARNING: CPU: 1 PID: 58 at kernel/rcu/sync.c:177 rcu_sync_dtor+0xcd/0x180 kernel/rcu/sync.c:177

CPU: 1 UID: 0 PID: 58 Comm: kworker/1:2 Not tainted 6.10.0-syzkaller-12562-g1722389b0d86 #0

Workqueue: events destroy_super_work

RIP: 0010:rcu_sync_dtor+0xcd/0x180 kernel/rcu/sync.c:177

Call Trace:

percpu_free_rwsem+0x41/0x80 kernel/locking/percpu-rwsem.c:42

destroy_super_work+0xec/0x130 fs/super.c:282

process_one_work kernel/workqueue.c:3231 [inline]

process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3312

worker_thread+0x86d/0xd40 kernel/workqueue.c:3390

kthread+0x2f0/0x390 kernel/kthread.c:389

ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147

ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

As Christian Brauner pointed out [1]: the root cause is f2fs sets

SB_RDONLY flag in internal function, rather than setting the flag

covered w/ sb->s_umount semaphore via remount procedure, then below

race condition causes this bug:

- freeze_super()

- sb_wait_write(sb, SB_FREEZE_WRITE)

- sb_wait_write(sb, SB_FREEZE_PAGEFAULT)

- sb_wait_write(sb, SB_FREEZE_FS)

- f2fs_handle_critical_error

- sb->s_flags |= SB_RDONLY

- thaw_super

- thaw_super_locked

- sb_rdonly() is true, so it skips

sb_freeze_unlock(sb, SB_FREEZE_FS)

- deactivate_locked_super

Since f2fs has almost the same logic as ext4 [2] when handling critical

error in filesystem if it mounts w/ errors=remount-ro option:

- set CP_ERROR_FLAG flag which indicates filesystem is stopped

- record errors to superblock

- set SB_RDONLY falg

Once we set CP_ERROR_FLAG flag, all writable interfaces can detect the

flag and stop any further updates on filesystem. So, it is safe to not

set SB_RDONLY flag, let's remove the logic and keep in line w/ ext4 [3].

[1] https://lore.kernel.org/all/20240729-himbeeren-funknetz-96e62f9c7aee@brauner

[2] https://lore.kernel.org/all/20240729132721.hxih6ehigadqf7wx@quack3

[3] https://lore.kernel.org/linux-ext4/[email protected]