[PATCH 0/6] File Sealing & memfd_create()

[Andy Lutomirski]

On Thu, Apr 10, 2014 at 1:32 PM, Theodore Ts'o <tytso at mit.edu> wrote:
> On Thu, Apr 10, 2014 at 12:14:27PM -0700, Andy Lutomirski wrote:
>>
>> This is the second time in a week that someone has asked for a way to
>> have a struct file (or struct inode or whatever) that can't be reopened
>> through /proc/pid/fd.  This should be quite easy to implement as a
>> separate feature.
>
> What I suggested on a different thread was to add the following new
> file descriptor flags, to join FD_CLOEXEC, which would be maniuplated
> using the F_GETFD and F_SETFD fcntl commands:
>
> FD_NOPROCFS     disallow being able to open the inode via /proc/<pid>/fd
>
> FD_NOPASSFD     disallow being able to pass the fd via a unix domain socket
>
> FD_LOCKFLAGS    if this bit is set, disallow any further changes of FD_CLOEXEC,
>                 FD_NOPROCFS, FD_NOPASSFD, and FD_LOCKFLAGS flags.
>
> Regardless of what else we might need to meet the use case for the
> proposed File Sealing API, I think this is a useful feature that could
> be used in many other contexts besides just the proposed
> memfd_create() use case.

It occurs to me that, before going nuts with these kinds of flags, it
may pay to just try to fix the /proc/self/fd issue for real -- we
could just make open("/proc/self/fd/3", O_RDWR) fail if fd 3 is
read-only.  That may be enough for the file sealing thing.

--Andy