JFS: more checks for invalid superblock

syzbot is feeding invalid superblock data to JFS for mount testing.
JFS does not check several of the fields -- just assumes that they
are good since the JFS_MAGIC and version fields are good.

In this case (syzbot reproducer), we have s_l2bsize == 0xda0c,
pad == 0xf045, and s_state == 0x50, all of which are invalid IMO.
Having s_l2bsize == 0xda0c causes this UBSAN warning:
  UBSAN: shift-out-of-bounds in fs/jfs/jfs_mount.c:373:25
  shift exponent -9716 is negative

s_l2bsize can be tested for correctness. pad can be tested for non-0
and punted. s_state can be tested for its valid values and punted.

Do those 3 tests and if any of them fails, report the superblock as
invalid/corrupt and let fsck handle it.

With this patch, chkSuper() says this when JFS_DEBUG is enabled:
  jfs_mount: Mount Failure: superblock is corrupt!
  Mount JFS Failure: -22
  jfs_mount failed w/return code = -22

The obvious problem with this method is that next week there could
be another syzbot test that uses different fields for invalid values,
this making this like a game of whack-a-mole.

syzkaller link: https://syzkaller.appspot.com/bug?extid=36315852ece4132ec193

Reported-by: syzbot+36315852ece4132ec193@syzkaller.appspotmail.com
Reported-by: kernel test robot <lkp@intel.com> # v2
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: jfs-discussion@lists.sourceforge.net

fs/jfs/jfs_filsys.h

@@ -268,5 +268,6 @@
 				 * fsck() must be run to repair
 				 */
 #define	FM_EXTENDFS 0x00000008	/* file system extendfs() in progress */
+#define	FM_STATE_MAX 0x0000000f	/* max value of s_state */
 
 #endif				/* _H_JFS_FILSYS */

fs/jfs/jfs_mount.c

@@ -37,6 +37,7 @@
 #include <linux/fs.h>
 #include <linux/buffer_head.h>
 #include <linux/blkdev.h>
+#include <linux/log2.h>
 
 #include "jfs_incore.h"
 #include "jfs_filsys.h"
@@ -366,6 +367,15 @@ static int chkSuper(struct super_block *sb)
 	sbi->bsize = bsize;
 	sbi->l2bsize = le16_to_cpu(j_sb->s_l2bsize);
 
+	/* check some fields for possible corruption */
+	if (sbi->l2bsize != ilog2((u32)bsize) ||
+	    j_sb->pad != 0 ||
+	    le32_to_cpu(j_sb->s_state) > FM_STATE_MAX) {
+		rc = -EINVAL;
+		jfs_err("jfs_mount: Mount Failure: superblock is corrupt!");
+		goto out;
+	}
+
 	/*
 	 * For now, ignore s_pbsize, l2bfactor.  All I/O going through buffer
 	 * cache.