linux-kernel - [PATCH AUTOSEL 5.10 21/56] JFS: more checks for invalid superblock

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Message-Id: <20210224125212.482485-21-sashal@kernel.org>
Date:   Wed, 24 Feb 2021 07:51:37 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        syzbot+36315852ece4132ec193@...kaller.appspotmail.com,
        kernel test robot <lkp@...el.com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        jfs-discussion@...ts.sourceforge.net,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.10 21/56] JFS: more checks for invalid superblock

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

[ Upstream commit 3bef198f1b17d1bb89260bad947ef084c0a2d1a6 ]

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: Sasha Levin <sashal@...nel.org>
---
 fs/jfs/jfs_filsys.h |  1 +
 fs/jfs/jfs_mount.c  | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/fs/jfs/jfs_filsys.h b/fs/jfs/jfs_filsys.h
index 1e899298f7f00..b5d702df7111a 100644
--- a/fs/jfs/jfs_filsys.h
+++ b/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 */
diff --git a/fs/jfs/jfs_mount.c b/fs/jfs/jfs_mount.c
index 2935d4c776ec7..5d7d7170c03c0 100644
--- a/fs/jfs/jfs_mount.c
+++ b/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.
-- 
2.27.0