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Date:   Thu, 17 Dec 2020 21:19:20 -0800
From:   Randy Dunlap <>
Cc:     Randy Dunlap <>,,
        Dave Kleikamp <>,
Subject: [PATCH] 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:

Signed-off-by: Randy Dunlap <>
Cc: Dave Kleikamp <>
 fs/jfs/jfs_filsys.h |    1 +
 fs/jfs/jfs_mount.c  |   10 ++++++++++
 2 files changed, 11 insertions(+)

--- lnx-510.orig/fs/jfs/jfs_mount.c
+++ lnx-510/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 *
 	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 ||
+	    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.
--- lnx-510.orig/fs/jfs/jfs_filsys.h
+++ lnx-510/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 */

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