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

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Date:   Mon, 10 Oct 2022 09:19:33 -0700
From:   Vishal Moola <vishal.moola@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+5867885efe39089b339b@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] UBSAN: array-index-out-of-bounds in truncate_inode_pages_range

On Sat, Oct 8, 2022 at 12:12 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Sep 01, 2022 at 04:24:59PM -0700, Andrew Morton wrote:
> > 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.
>
> NTFS is involved.  I stopped looking at that point; it seems to be
> riddled with buffer overflows.
>
> > I must say, the the code looks rather hacky.  Isn't there a more
> > type-friendly way of doing this?
>
> Looking at the three callers, they all want to advance index.  We
> should probably pass &index instead of index and have find_lock_entries
> advance it for them.
>
> Vishal, want to take this on?

Yup! I'll do that.