linux-kernel - Re: [PATCH 1/2] fcntl: add fcntl(F_CHECK_ORIGINAL

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Message-ID: <202309011133.10D66D0785@keescook>
Date:   Fri, 1 Sep 2023 11:34:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Michał Cłapiński <mclapinski@...gle.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jeff Xu <jeffxu@...gle.com>, Aleksa Sarai <cyphar@...har.com>,
        Daniel Verkamp <dverkamp@...omium.org>,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] fcntl: add fcntl(F_CHECK_ORIGINAL_MEMFD)

On Fri, Sep 01, 2023 at 04:50:53PM +0200, Michał Cłapiński wrote:
> On Fri, Sep 1, 2023 at 2:56 PM Christian Brauner <brauner@...nel.org> wrote:
> >
> > On Thu, Aug 31, 2023 at 10:36:46PM +0200, Michal Clapinski wrote:
> > > Add a way to check if an fd points to the memfd's original open fd
> > > (the one created by memfd_create).
> > > Useful because only the original open fd can be both writable and
> > > executable.
> > >
> > > Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
> > > ---
> > >  fs/fcntl.c                 | 3 +++
> > >  include/uapi/linux/fcntl.h | 9 +++++++++
> > >  2 files changed, 12 insertions(+)
> > >
> > > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > > index e871009f6c88..301527e07a4d 100644
> > > --- a/fs/fcntl.c
> > > +++ b/fs/fcntl.c
> > > @@ -419,6 +419,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
> > >       case F_SET_RW_HINT:
> > >               err = fcntl_rw_hint(filp, cmd, arg);
> > >               break;
> > > +     case F_CHECK_ORIGINAL_MEMFD:
> > > +             err = !(filp->f_mode & FMODE_WRITER);
> > > +             break;
> >
> > Honestly, make this an ioctl on memfds. This is so specific that it
> > really doesn't belong into fcntl().
> 
> I've never touched ioctls but if I'm correct, I can't just add it to
> memfd. I would have to add it to the underlying fs, so hugetlbfs and
> shmem (which I think can be defined as ramfs so also there). File
> sealing fcntl is already memfd specific. Are you sure ioctl will be a
> better idea?

Does this check "mean" anything for other files? Because if it's
generically useful (and got renamed) it maybe would be right for
fcntl...

-- 
Kees Cook