Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

From: Joel Fernandes <joel@joelfernandes.org>
To: Michael Tirado <mtirado418@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	jreck@google.com, john.stultz@linaro.org, tkjos@google.com,
	gregkh@linuxfoundation.org, hch@infradead.org,
	viro@zeniv.linux.org.uk,
	Andrew Morton <akpm@linux-foundation.org>,
	dancol@google.com, bfields@fieldses.org, jlayton@kernel.org,
	khalid.aziz@oracle.com, Lei.Yang@windriver.com,
	linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org, marcandre.lureau@redhat.com,
	mike.kravetz@oracle.com, minchan@kernel.org, shuah@kernel.org,
	valdis.kletnieks@vt.edu, hughd@google.com,
	linux-api@vger.kernel.org
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
Date: Fri, 9 Nov 2018 17:49:13 -0800	[thread overview]
Message-ID: <20181110014913.GA202500@google.com> (raw)
In-Reply-To: <CAMkWEXOLJ=ymbVjQfA2MD8XA7Y9Lu3ByJYUY-JvpnYKJ5gkY1w@mail.gmail.com>

On Fri, Nov 09, 2018 at 08:02:14PM +0000, Michael Tirado wrote:
[...]
> > > That aside: I wonder whether a better API would be something that
> > > allows you to create a new readonly file descriptor, instead of
> > > fiddling with the writability of an existing fd.
> >
> > Every now and then I try to write a patch to prevent using proc to reopen
> > a file with greater permission than the original open.
> >
> > I like your idea to have a clean way to reopen a a memfd with reduced
> > permissions. But I would make it a syscall instead and maybe make it only
> > work for memfd at first.  And the proc issue would need to be fixed, too.
> 
> IMO the best solution would handle the issue at memfd creation time by
> removing the race condition.

I agree, this is another idea I'm exploring. We could add a new .open
callback to shmem_file_operations and check for seals there.

thanks,

 - Joel