linux-kernel - Re: [syzbot] UBSAN: array-index-out-of-bounds in truncate_inode_pages

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Message-Id: <20220901162459.431c49b3925e99ddb448e1b3@linux-foundation.org>
Date:   Thu, 1 Sep 2022 16:24:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     syzbot <syzbot+5867885efe39089b339b@...kaller.appspotmail.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        willy@...radead.org
Subject: Re: [syzbot] UBSAN: array-index-out-of-bounds in
 truncate_inode_pages_range

On Wed, 31 Aug 2022 17:13:36 -0700 syzbot <syzbot+5867885efe39089b339b@...kaller.appspotmail.com> wrote:

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    89b749d8552d Merge tag 'fbdev-for-6.0-rc3' of git://git.ke..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14b9661b080000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=911efaff115942bb
> dashboard link: https://syzkaller.appspot.com/bug?extid=5867885efe39089b339b
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> userspace arch: i386
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+5867885efe39089b339b@...kaller.appspotmail.com
> 
> ntfs3: loop0: Different NTFS' sector size (1024) and media sector size (512)
> ntfs3: loop0: RAW NTFS volume: Filesystem size 0.00 Gb > volume size 0.00 Gb. Mount in read-only
> ================================================================================
> UBSAN: array-index-out-of-bounds in mm/truncate.c:366:18
> index 254 is out of range for type 'long unsigned int [15]'

That's

		index = indices[folio_batch_count(&fbatch) - 1] + 1;

I looked.  I see no way in which fbatch.nr got a value of 255.


I must say, the the code looks rather hacky.  Isn't there a more
type-friendly way of doing this?