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Date:   Fri, 18 Dec 2020 14:31:50 -0600
From:   Dave Kleikamp <dave.kleikamp@...cle.com>
To:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org
Cc:     syzbot+36315852ece4132ec193@...kaller.appspotmail.com,
        kernel test robot <lkp@...el.com>,
        Dave Kleikamp <shaggy@...nel.org>,
        jfs-discussion@...ts.sourceforge.net
Subject: Re: [PATCH v2] JFS: more checks for invalid superblock

Thanks! This looks good and reasonable. I'll try to get it pushed out to 
-next in the next few days.

Shaggy

On 12/18/20 2:17 PM, Randy Dunlap wrote:
> 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>
> Cc: Dave Kleikamp <shaggy@...nel.org>
> Cc: jfs-discussion@...ts.sourceforge.net
> ---
> v2: fix sparse __le32 warning (lkp robot)
> 
>   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 ||
> +	    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.
> --- 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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