Re: [PATCH 08/11] vfs: push EXDEV check down into ->remap_file_range

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Dec 03, 2018 at 01:04:07PM +0200, Amir Goldstein wrote:
> On Mon, Dec 3, 2018 at 10:34 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > before we can enable cross-device copies into copy_file_range(),
> > we have to ensure that ->remap_file_range() implemenations will
> > correctly reject attempts to do cross filesystem clones. Currently
> 
> But you only fixed remap_file_range() implemenations of xfs and ocfs2...
> 
> > these checks are done above calls to ->remap_file_range(), but
> > we need to drive them inwards so that we get EXDEV protection for all
> > callers of ->remap_file_range().
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> >  fs/read_write.c | 21 +++++++++++++--------
> >  1 file changed, 13 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/read_write.c b/fs/read_write.c
> > index 3288db1d5f21..174cf92eea1d 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -1909,6 +1909,19 @@ int generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
> >         bool same_inode = (inode_in == inode_out);
> >         int ret;
> >
> > +       /*
> > +        * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
> > +        * the same mount. Practically, they only need to be on the same file
> > +        * system. We check this here rather than at the ioctl layers because
> > +        * this is effectively a limitation of the fielsystem implementations,
> > +        * not so much the API itself. Further, ->remap_file_range() can be
> > +        * called from syscalls that don't have cross device copy restrictions
> > +        * (such as copy_file_range()) and so we need to catch them before we
> > +        * do any damage.
> > +        */
> > +       if (inode_in->i_sb != inode_out->i_sb)
> > +               return -EXDEV;
> > +
> >         /* Don't touch certain kinds of inodes */
> >         if (IS_IMMUTABLE(inode_out))
> >                 return -EPERM;
> > @@ -2013,14 +2026,6 @@ loff_t do_clone_file_range(struct file *file_in, loff_t pos_in,
> >         if (!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode))
> >                 return -EINVAL;
> >
> > -       /*
> > -        * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
> > -        * the same mount. Practically, they only need to be on the same file
> > -        * system.
> > -        */
> > -       if (inode_in->i_sb != inode_out->i_sb)
> > -               return -EXDEV;
> > -
> 

I think this is sort of backwards -- the checks should stay in
do_clone_file_range, and vfs_copy_file_range should be calling that
instead of directly calling ->remap_range():

vfs_copy_file_range()
{
	file_start_write(...);
	ret = do_clone_file_range(...);
	if (ret > 0)
		return ret;
	ret = do_copy_file_range(...);
	file_end_write(...);
	return ret;
}

> That leaves {nfs42,cifs,btrfs}_remap_file_range() exposed to passing
> files not of their own fs type let alone same sb when do_clone_file_range()
> is called from ovl_copy_up_data().

...and then I think this problem goes away.

--D

> Thanks,
> Amir.