[PATCH] fs/minix: Improve validity checking of superblock

[Juntong Deng]

According to the Minix source code, s_imap_blocks, s_zmap_blocks,
s_ninodes, s_zones, s_firstdatazone, and s_log_zone_size should
be checked for validity when reading superblocks.

The following is the content of minix/fs/mfs/super.c:332

  /* Make a few basic checks to see if super block looks reasonable. */
  if (sp->s_imap_blocks < 1 || sp->s_zmap_blocks < 1
				|| sp->s_ninodes < 1 || sp->s_zones < 1
				|| sp->s_firstdatazone <= 4
				|| sp->s_firstdatazone >= sp->s_zones
				|| (unsigned) sp->s_log_zone_size > 4) {
	printf("not enough imap or zone map blocks, \n");
	printf("or not enough inodes, or not enough zones, \n"
		"or invalid first data zone, or zone size too large\n");
	return(EINVAL);
  }

This patch improve the validity checking of superblock based on the
Minix source code above.

Since the validity of s_log_zone_size is not currently checked,
this can lead to errors when s_log_zone_size is subsequently used
as a shift exponent.

The following are related bugs reported by Syzbot:

UBSAN: shift-out-of-bounds in fs/minix/bitmap.c:103:3
shift exponent 34 is too large for 32-bit type 'unsigned int'

UBSAN: shift-out-of-bounds in fs/minix/inode.c:380:57
shift exponent 65510 is too large for 64-bit type 'long unsigned int'

Reported-by: syzbot+2f142b57f2af27974fda@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=2f142b57f2af27974fda
Reported-by: syzbot+5ad0824204c7bf9b67f2@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=5ad0824204c7bf9b67f2
Signed-off-by: Juntong Deng <juntong.deng@xxxxxxxxxxx>
---
 fs/minix/inode.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/minix/inode.c b/fs/minix/inode.c
index df575473c1cc..84c2c6e77d1d 100644
--- a/fs/minix/inode.c
+++ b/fs/minix/inode.c
@@ -154,7 +154,11 @@ static bool minix_check_superblock(struct super_block *sb)
 {
 	struct minix_sb_info *sbi = minix_sb(sb);
 
-	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
+	if (sbi->s_imap_blocks < 1 || sbi->s_zmap_blocks < 1 ||
+		sbi->s_ninodes < 1 || sbi->s_nzones < 1 ||
+		sbi->s_firstdatazone <= 4 ||
+		sbi->s_firstdatazone >= sbi->s_nzones ||
+		sbi->s_log_zone_size > 4)
 		return false;
 
 	/*
-- 
2.39.2