Re: [syzbot] UBSAN: array-index-out-of-bounds in truncate_inode_pages_range

[Dmitry Vyukov]

On Fri, 2 Sept 2022 at 01:25, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 31 Aug 2022 17:13:36 -0700 syzbot <syzbot+5867885efe39089b339b@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > 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?

I don't see how this can happen either. Also can't reproduce.
It's happened only once so far, so maybe some silent memory corruption.
Let's wait for more crashes/reproducer, or otherwise syzbot will
auto-close it after some time.