[PATCH] fs: f2fs: fix potential shift-out-of-bounds error in sanity_check_raw_super()

[Anant Thazhemadam]

In sanity_check_raw_super(), if
1 << le32_to_cpu(raw_super->log_blocksize) != F2FS_BLKSIZE, then the
block size is deemed to be invalid.

syzbot triggered a shift-out-of-bounds bug by assigning a value of 59 to
le32_to_cpu(raw_super->log_blocksize).
Although the value assigned itself isn't of much significance, this goes
to show that even if the block size is invalid,
le32_to_cpu(raw_super->log_blocksize) can be potentially evaluated to a
value for which the shift exponent becomes too large for the unsigned
int.

Since 1 << le32_to_cpu(raw_super->log_blocksize) must be = 4096 for a
valid block size, le32_to_cpu(raw_super->log_blocksize) must equal 12.
Replacing the existing check with the more direct sanity check
resolves this bug.

Reported-by: syzbot+ca9a785f8ac472085...@syzkaller.appspotmail.com
Tested-by: syzbot+ca9a785f8ac472085...@syzkaller.appspotmail.com
Signed-off-by: Anant Thazhemadam <anant.thazhema...@gmail.com>
---
 fs/f2fs/super.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 33808c397580..4bc7372af43f 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2775,7 +2775,6 @@ static int sanity_check_raw_super(struct f2fs_sb_info 
*sbi,
        block_t total_sections, blocks_per_seg;
        struct f2fs_super_block *raw_super = (struct f2fs_super_block *)
                                        (bh->b_data + F2FS_SUPER_OFFSET);
-       unsigned int blocksize;
        size_t crc_offset = 0;
        __u32 crc = 0;
 
@@ -2802,10 +2801,8 @@ static int sanity_check_raw_super(struct f2fs_sb_info 
*sbi,
        }
 
        /* Currently, support only 4KB block size */
-       blocksize = 1 << le32_to_cpu(raw_super->log_blocksize);
-       if (blocksize != F2FS_BLKSIZE) {
-               f2fs_info(sbi, "Invalid blocksize (%u), supports only 4KB",
-                         blocksize);
+       if (le32_to_cpu(raw_super->log_blocksize) != 12) {
+               f2fs_info(sbi, "Invalid blocksize. Only 4KB supported");
                return -EFSCORRUPTED;
        }
 
-- 
2.25.1