linux-kernel - Re: kernel BUG at mm/hugetlb.c:LINE!

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Date:   Wed, 20 May 2020 13:20:27 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Colin Walters <walters@...bum.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: kernel BUG at mm/hugetlb.c:LINE!

On Tue, May 19, 2020 at 2:35 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 5/18/20 4:41 PM, Colin Walters wrote:
> >
> > On Tue, May 12, 2020, at 11:04 AM, Miklos Szeredi wrote:
> >
> >>> However, in this syzbot test case the 'file' is in an overlayfs filesystem
> >>> created as follows:
> >>>
> >>> mkdir("./file0", 000)                   = 0
> >>> mount(NULL, "./file0", "hugetlbfs", MS_MANDLOCK|MS_POSIXACL, NULL) = 0
> >>> chdir("./file0")                        = 0
> >>> mkdir("./file1", 000)                   = 0
> >>> mkdir("./bus", 000)                     = 0
> >>> mkdir("./file0", 000)                   = 0
> >>> mount("\177ELF\2\1\1", "./bus", "overlay", 0, "lowerdir=./bus,workdir=./file1,u"...) = 0
> >
> > Is there any actual valid use case for mounting an overlayfs on top of hugetlbfs?  I can't think of one.  Why isn't the response to this to instead only allow mounting overlayfs on top of basically a set of whitelisted filesystems?
> >
>
> I can not think of a use case.  I'll let Miklos comment on adding whitelist
> capability to overlayfs.

I've not heard of overlayfs being used over hugetlbfs.

Overlayfs on tmpfs is definitely used, I guess without hugepages.
But if we'd want to allow tmpfs _without_ hugepages but not tmpfs
_with_ hugepages, then we can't just whitelist based on filesystem
type, but need to look at mount options as well.  Which isn't really a
clean solution either.

> IMO - This BUG/report revealed two issues.  First is the BUG by mmap'ing
> a hugetlbfs file on overlayfs.  The other is that core mmap code will skip
> any filesystem specific get_unmapped area routine if on a union/overlay.
> My patch fixes both, but if we go with a whitelist approach and don't allow
> hugetlbfs I think we still need to address the filesystem specific
> get_unmapped area issue.  That is easy enough to do by adding a routine to
> overlayfs which calls the routine for the underlying fs.

I think the two are strongly related:  get_unmapped_area() adjusts the
address alignment, and the is_file_hugepages() call in
ksys_mmap_pgoff() adjusts the length alignment.

Is there any other purpose for which  f_op->get_unmapped_area() is
used by a filesystem?

Thanks,
Miklos