f2fs: enhance sanity_check_raw_super() to avoid potential overflows

In order to avoid the below overflow issue, we should have checked the
boundaries in superblock before reaching out to allocation. As Linus suggested,
the right place should be sanity_check_raw_super().

Dr Silvio Cesare of InfoSect reported:

There are integer overflows with using the cp_payload superblock field in the
f2fs filesystem potentially leading to memory corruption.

include/linux/f2fs_fs.h

struct f2fs_super_block {
...
        __le32 cp_payload;

fs/f2fs/f2fs.h

typedef u32 block_t;    /*
                         * should not change u32, since it is the on-disk block
                         * address format, __le32.
                         */
...

static inline block_t __cp_payload(struct f2fs_sb_info *sbi)
{
        return le32_to_cpu(F2FS_RAW_SUPER(sbi)->cp_payload);
}

fs/f2fs/checkpoint.c

        block_t start_blk, orphan_blocks, i, j;
...
        start_blk = __start_cp_addr(sbi) + 1 + __cp_payload(sbi);
        orphan_blocks = __start_sum_addr(sbi) - 1 - __cp_payload(sbi);

+++ integer overflows

...
        unsigned int cp_blks = 1 + __cp_payload(sbi);
...
        sbi->ckpt = kzalloc(cp_blks * blk_size, GFP_KERNEL);

+++ integer overflow leading to incorrect heap allocation.

        int cp_payload_blks = __cp_payload(sbi);
...
        ckpt->cp_pack_start_sum = cpu_to_le32(1 + cp_payload_blks +
                        orphan_blocks);

+++ sign bug and integer overflow

...
        for (i = 1; i < 1 + cp_payload_blks; i++)

+++ integer overflow

...

      sbi->max_orphans = (sbi->blocks_per_seg - F2FS_CP_PACKS -
                        NR_CURSEG_TYPE - __cp_payload(sbi)) *
                                F2FS_ORPHANS_PER_BLOCK;

+++ integer overflow

Reported-by: Greg KH <greg@kroah.com>
Reported-by: Silvio Cesare <silvio.cesare@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>