linux-kernel - [PATCH 4.9 18/41] JFS: more checks for invalid superblock

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Message-Id: <20210305120852.182320874@linuxfoundation.org>
Date:   Fri,  5 Mar 2021 13:22:25 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        syzbot+36315852ece4132ec193@...kaller.appspotmail.com,
        Randy Dunlap <rdunlap@...radead.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        jfs-discussion@...ts.sourceforge.net,
        kernel test robot <lkp@...el.com>
Subject: [PATCH 4.9 18/41] JFS: more checks for invalid superblock

From: Randy Dunlap <rdunlap@...radead.org>

commit 3bef198f1b17d1bb89260bad947ef084c0a2d1a6 upstream.

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@...kaller.appspotmail.com
Reported-by: kernel test robot <lkp@...el.com> # v2
Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@...cle.com>
Cc: jfs-discussion@...ts.sourceforge.net
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/jfs/jfs_filsys.h |    1 +
 fs/jfs/jfs_mount.c  |   10 ++++++++++
 2 files changed, 11 insertions(+)

--- a/fs/jfs/jfs_filsys.h
+++ b/fs/jfs/jfs_filsys.h
@@ -281,5 +281,6 @@
 				 * 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 */
--- a/fs/jfs/jfs_mount.c
+++ b/fs/jfs/jfs_mount.c
@@ -49,6 +49,7 @@

 #include <linux/fs.h>
 #include <linux/buffer_head.h>
+#include <linux/log2.h>

 #include "jfs_incore.h"
 #include "jfs_filsys.h"
@@ -378,6 +379,15 @@ 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 ||
+	    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.