f2fs: don't reset unchangable mount option in f2fs_remount()

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

f2fs: don't reset unchangable mount option in f2fs_remount()

syzbot reports a bug as below:

general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] PREEMPT SMP KASAN

RIP: 0010:__lock_acquire+0x69/0x2000 kernel/locking/lockdep.c:4942

Call Trace:

lock_acquire+0x1e3/0x520 kernel/locking/lockdep.c:5691

__raw_write_lock include/linux/rwlock_api_smp.h:209 [inline]

_raw_write_lock+0x2e/0x40 kernel/locking/spinlock.c:300

__drop_extent_tree+0x3ac/0x660 fs/f2fs/extent_cache.c:1100

f2fs_drop_extent_tree+0x17/0x30 fs/f2fs/extent_cache.c:1116

f2fs_insert_range+0x2d5/0x3c0 fs/f2fs/file.c:1664

f2fs_fallocate+0x4e4/0x6d0 fs/f2fs/file.c:1838

vfs_fallocate+0x54b/0x6b0 fs/open.c:324

ksys_fallocate fs/open.c:347 [inline]

__do_sys_fallocate fs/open.c:355 [inline]

__se_sys_fallocate fs/open.c:353 [inline]

__x64_sys_fallocate+0xbd/0x100 fs/open.c:353

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

The root cause is race condition as below:

- since it tries to remount rw filesystem, so that do_remount won't

call sb_prepare_remount_readonly to block fallocate, there may be race

condition in between remount and fallocate.

- in f2fs_remount(), default_options() will reset mount option to default

one, and then update it based on result of parse_options(), so there is

a hole which race condition can happen.

Thread A Thread B

- f2fs_fill_super

- parse_options

- clear_opt(READ_EXTENT_CACHE)

- f2fs_remount

- default_options

- set_opt(READ_EXTENT_CACHE)

- f2fs_fallocate

- f2fs_insert_range

- f2fs_drop_extent_tree

- __drop_extent_tree

- __may_extent_tree

- test_opt(READ_EXTENT_CACHE) return true

- write_lock(&et->lock) access NULL pointer

- parse_options

- clear_opt(READ_EXTENT_CACHE)