Describe CVE-2021-47007.
What is CVE-2021-47007?
In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix panic during f2fs_resize_fs()

f2fs_resize_fs() hangs in below callstack with testcase:
- mkfs 16GB image & mount image
- dd 8GB fileA
- dd 8GB fileB
- sync
- rm fileA
- sync
- resize filesystem to 8GB

kernel BUG at segment.c:2484!
Call Trace:
 allocate_segment_by_default+0x92/0xf0 [f2fs]
 f2fs_allocate_data_block+0x44b/0x7e0 [f2fs]
 do_write_page+0x5a/0x110 [f2fs]
 f2fs_outplace_write_data+0x55/0x100 [f2fs]
 f2fs_do_write_data_page+0x392/0x850 [f2fs]
 move_data_page+0x233/0x320 [f2fs]
 do_garbage_collect+0x14d9/0x1660 [f2fs]
 free_segment_range+0x1f7/0x310 [f2fs]
 f2fs_resize_fs+0x118/0x330 [f2fs]
 __f2fs_ioctl+0x487/0x3680 [f2fs]
 __x64_sys_ioctl+0x8e/0xd0
 do_syscall_64+0x33/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The root cause is we forgot to check that whether we have enough space
in resized filesystem to store all valid blocks in before-resizing
filesystem, then allocator will run out-of-space during block migration
in free_segment_range()..