linux-kernel - Re: [PATCH] fs: jfs: fix shift-out-of-bounds in dbDiscardAG

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Date:   Mon, 24 Oct 2022 16:05:30 -0500
From:   Dave Kleikamp <dave.kleikamp@...cle.com>
To:     Hoi Pok Wu <wuhoipok@...il.com>
Cc:     jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        syzbot+f0e0fcf3cd1047ae60ad@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] fs: jfs: fix shift-out-of-bounds in dbDiscardAG

On 10/19/22 4:38AM, Hoi Pok Wu wrote:
> Bug reported by syzbot, UBSAN: array-index-out-of-bounds in dbDiscardAG.
> Fixed by simply rearranging agno_end smaller than MAXAG. However, I wonder if it
> is a better idea to return -EINVAL, rejecting user input. Thank you.

I don't think the problem is simply that the user input is bad, the 
range is already limited:

         if (end >= bmp->db_mapsize)
                 end = bmp->db_mapsize - 1;

It seems syzbot corrupted some on-disk data structure to trigger this 
failure. It may be best to call jfs_error() and bail out.

Thanks,
Shaggy

> 
> Reported-by: syzbot+f0e0fcf3cd1047ae60ad@...kaller.appspotmail.com
> Signed-off-by: Hoi Pok Wu <wuhoipok@...il.com>
> ---
>   fs/jfs/jfs_discard.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/fs/jfs/jfs_discard.c b/fs/jfs/jfs_discard.c
> index 575cb2ba74fc..3101eaf3098a 100644
> --- a/fs/jfs/jfs_discard.c
> +++ b/fs/jfs/jfs_discard.c
> @@ -96,6 +96,10 @@ int jfs_ioc_trim(struct inode *ip, struct fstrim_range *range)
>   	 */
>   	agno = BLKTOAG(start, JFS_SBI(ip->i_sb));
>   	agno_end = BLKTOAG(end, JFS_SBI(ip->i_sb));
> +
> +	if (agno_end >= MAXAG)
> +		agno_end = MAXAG - 1;
> +
>   	while (agno <= agno_end) {
>   		trimmed += dbDiscardAG(ip, agno, minlen);
>   		agno++;