nilfs2: fix shift-out-of-bounds/overflow in nilfs_sb2_bad_offset()

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

nilfs2: fix shift-out-of-bounds/overflow in nilfs_sb2_bad_offset()

Patch series "nilfs2: fix UBSAN shift-out-of-bounds warnings on mount

time".

The first patch fixes a bug reported by syzbot, and the second one fixes

the remaining bug of the same kind. Although they are triggered by the

same super block data anomaly, I divided it into the above two because the

details of the issues and how to fix it are different.

Both are required to eliminate the shift-out-of-bounds issues at mount

time.

This patch (of 2):

If the block size exponent information written in an on-disk superblock is

corrupted, nilfs_sb2_bad_offset helper function can trigger

shift-out-of-bounds warning followed by a kernel panic (if panic_on_warn

is set):

shift exponent 38983 is too large for 64-bit type 'unsigned long long'

Call Trace:

<TASK>

__dump_stack lib/dump_stack.c:88 [inline]

dump_stack_lvl+0x1b1/0x28e lib/dump_stack.c:106

ubsan_epilogue lib/ubsan.c:151 [inline]

__ubsan_handle_shift_out_of_bounds+0x33d/0x3b0 lib/ubsan.c:322

nilfs_sb2_bad_offset fs/nilfs2/the_nilfs.c:449 [inline]

nilfs_load_super_block+0xdf5/0xe00 fs/nilfs2/the_nilfs.c:523

init_nilfs+0xb7/0x7d0 fs/nilfs2/the_nilfs.c:577

nilfs_fill_super+0xb1/0x5d0 fs/nilfs2/super.c:1047

nilfs_mount+0x613/0x9b0 fs/nilfs2/super.c:1317

...

In addition, since nilfs_sb2_bad_offset() performs multiplication without

considering the upper bound, the computation may overflow if the disk

layout parameters are not normal.

This fixes these issues by inserting preliminary sanity checks for those

parameters and by converting the comparison from one involving

multiplication and left bit-shifting to one using division and right

bit-shifting.