Re: [PATCH v2] vfs: prevent copy_file_range to copy across devices

[Amir Goldstein]

On Tue, Feb 16, 2021 at 6:41 PM Luis Henriques <lhenriques@xxxxxxx> wrote:
>
> Amir Goldstein <amir73il@xxxxxxxxx> writes:
>
> >> Ugh.  And I guess overlayfs may have a similar problem.
> >
> > Not exactly.
> > Generally speaking, overlayfs should call vfs_copy_file_range()
> > with the flags it got from layer above, so if called from nfsd it
> > will allow cross fs copy and when called from syscall it won't.
> >
> > There are some corner cases where overlayfs could benefit from
> > COPY_FILE_SPLICE (e.g. copy from lower file to upper file), but
> > let's leave those for now. Just leave overlayfs code as is.
>
> Got it, thanks for clarifying.
>
> >> > This is easy to solve with a flag COPY_FILE_SPLICE (or something) that
> >> > is internal to kernel users.
> >> >
> >> > FWIW, you may want to look at the loop in ovl_copy_up_data()
> >> > for improvements to nfsd_copy_file_range().
> >> >
> >> > We can move the check out to copy_file_range syscall:
> >> >
> >> >         if (flags != 0)
> >> >                 return -EINVAL;
> >> >
> >> > Leave the fallback from all filesystems and check for the
> >> > COPY_FILE_SPLICE flag inside generic_copy_file_range().
> >>
> >> Ok, the diff bellow is just to make sure I understood your suggestion.
> >>
> >> The patch will also need to:
> >>
> >>  - change nfs and overlayfs calls to vfs_copy_file_range() so that they
> >>    use the new flag.
> >>
> >>  - check flags in generic_copy_file_checks() to make sure only valid flags
> >>    are used (COPY_FILE_SPLICE at the moment).
> >>
> >> Also, where should this flag be defined?  include/uapi/linux/fs.h?
> >
> > Grep for REMAP_FILE_
> > Same header file, same Documentation rst file.
> >
> >>
> >> Cheers,
> >> --
> >> Luis
> >>
> >> diff --git a/fs/read_write.c b/fs/read_write.c
> >> index 75f764b43418..341d315d2a96 100644
> >> --- a/fs/read_write.c
> >> +++ b/fs/read_write.c
> >> @@ -1383,6 +1383,13 @@ ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
> >>                                 struct file *file_out, loff_t pos_out,
> >>                                 size_t len, unsigned int flags)
> >>  {
> >> +       if (!(flags & COPY_FILE_SPLICE)) {
> >> +               if (!file_out->f_op->copy_file_range)
> >> +                       return -EOPNOTSUPP;
> >> +               else if (file_out->f_op->copy_file_range !=
> >> +                        file_in->f_op->copy_file_range)
> >> +                       return -EXDEV;
> >> +       }
> >
> > That looks strange, because you are duplicating the logic in
> > do_copy_file_range(). Maybe better:
> >
> > if (WARN_ON_ONCE(flags & ~COPY_FILE_SPLICE))
> >         return -EINVAL;
> > if (flags & COPY_FILE_SPLICE)
> >        return do_splice_direct(file_in, &pos_in, file_out, &pos_out,
> >                                  len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);
>
> My initial reasoning for duplicating the logic in do_copy_file_range() was
> to allow the generic_copy_file_range() callers to be left unmodified and
> allow the filesystems to default to this implementation.
>
> With this change, I guess that the calls to generic_copy_file_range() from
> the different filesystems can be dropped, as in my initial patch, as they
> will always get -EINVAL.  The other option would be to set the
> COPY_FILE_SPLICE flag in those calls, but that would get us back to the
> problem we're trying to solve.

I don't understand the problem.

What exactly is wrong with the code I suggested?
Why should any filesystem be changed?

Maybe I am missing something.

Thanks,
Amir.