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From:   Steve French <smfrench@gmail.com>
Date:   Tue, 16 Feb 2021 13:31:21 -0600
Message-ID: <CAH2r5mvybG3mRUfHUub9B+nk5WrQ5Fvzu5pZ+ZynqZg4c4ct2A@mail.gmail.com>
Subject: Re: [PATCH v2] vfs: prevent copy_file_range to copy across devices
To:     Anna Schumaker <anna.schumaker@netapp.com>
Cc:     Amir Goldstein <amir73il@gmail.com>,
        Luis Henriques <lhenriques@suse.de>,
        Trond Myklebust <trondmy@hammerspace.com>,
        "samba-technical@lists.samba.org" <samba-technical@lists.samba.org>,
        "drinkcat@chromium.org" <drinkcat@chromium.org>,
        "iant@google.com" <iant@google.com>,
        "linux-cifs@vger.kernel.org" <linux-cifs@vger.kernel.org>,
        "darrick.wong@oracle.com" <darrick.wong@oracle.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jlayton@kernel.org" <jlayton@kernel.org>,
        "llozano@chromium.org" <llozano@chromium.org>,
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        "sfrench@samba.org" <sfrench@samba.org>,
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On Tue, Feb 16, 2021 at 1:29 PM Anna Schumaker
<anna.schumaker@netapp.com> wrote:
>
> On Tue, Feb 16, 2021 at 2:22 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Tue, Feb 16, 2021 at 8:54 PM Luis Henriques <lhenriques@suse.de> wrote:
> > >
> > > Amir Goldstein <amir73il@gmail.com> writes:
> > >
> > > > On Tue, Feb 16, 2021 at 6:41 PM Luis Henriques <lhenriques@suse.de> wrote:
> > > >>
> > > >> Amir Goldstein <amir73il@gmail.com> 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.
> > >
> > > Ok, I have to do a full brain reboot and start all over.
> > >
> > > Before that, I picked the code you suggested and tested it.  I've mounted
> > > a cephfs filesystem and used xfs_io to execute a 'copy_range' command
> > > using /sys/kernel/debug/sched_features as source.  The result was a
> > > 0-sized file in cephfs.  And the reason is thevfs_copy_file_range()
> > > early exit in:
> > >
> > >         if (len == 0)
> > >                 return 0;
> > >
> > > 'len' is set in generic_copy_file_checks().
> >
> > Good point.. I guess we will need to do all the checks earlier in
> > generic_copy_file_checks() including the logic of:
> >
> >         if (file_in->f_op->remap_file_range &&
> >             file_inode(file_in)->i_sb == file_inode(file_out)->i_sb)
> >
> >
> > >
> > > This means that we're not solving the original problem anymore (probably
> > > since v1 of this patch, haven't checked).
> > >
> > > Also, re-reading Trond's emails, I read: "... also disallowing the copy
> > > from, say, an XFS formatted partition to an ext4 partition".  Isn't that
> > > *exactly* what we're trying to do here?  I.e. _prevent_ these copies from
> > > happening so that tracefs files can't be CFR'ed?
> > >
> >
> > We want to address the report which means calls coming from
> > copy_file_range() syscall.
> >
> > Trond's use case is vfs_copy_file_range() coming from nfsd.
> > When he writes about copy from XFS to ext4, he means an
> > NFS client is issuing server side copy (on same or different NFS mounts)
> > and the NFS server is executing nfsd_copy_file_range() on a source
> > file that happens to be on XFS and destination happens to be on ext4.
>
> NFS also supports a server-to-server copy where the destination server
> mounts the source server and reads the data to be copied. Please don't
> break that either :)

This is a case we will eventually need to support for cifs (SMB3) as well.


-- 
Thanks,

Steve