CVE-2025-38218

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

f2fs: fix to do sanity check on sit_bitmap_size

w/ below testcase, resize will generate a corrupted image which
contains inconsistent metadata, so when mounting such image, it
will trigger kernel panic:

touch img
truncate -s $((512*1024*1024*1024)) img
mkfs.f2fs -f img $((256*1024*1024))
resize.f2fs -s -i img -t $((1024*1024*1024))
mount img /mnt/f2fs

------------[ cut here ]------------
kernel BUG at fs/f2fs/segment.h:863!
Oops: invalid opcode: 0000 [#1] SMP PTI
CPU: 11 UID: 0 PID: 3922 Comm: mount Not tainted 6.15.0-rc1+ #191 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:f2fs_ra_meta_pages+0x47c/0x490

Call Trace:
 f2fs_build_segment_manager+0x11c3/0x2600
 f2fs_fill_super+0xe97/0x2840
 mount_bdev+0xf4/0x140
 legacy_get_tree+0x2b/0x50
 vfs_get_tree+0x29/0xd0
 path_mount+0x487/0xaf0
 __x64_sys_mount+0x116/0x150
 do_syscall_64+0x82/0x190
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fdbfde1bcfe

The reaseon is:

sit_i->bitmap_size is 192, so size of sit bitmap is 192*8=1536, at maximum
there are 1536 sit blocks, however MAIN_SEGS is 261893, so that sit_blk_cnt
is 4762, build_sit_entries() -> current_sit_addr() tries to access
out-of-boundary in sit_bitmap at offset from [1536, 4762), once sit_bitmap
and sit_bitmap_mirror is not the same, it will trigger f2fs_bug_on().

Let's add sanity check in f2fs_sanity_check_ckpt() to avoid panic.