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

[Andy Lutomirski]

On 03/20/2014 09:38 AM, tytso at mit.edu wrote:
> On Thu, Mar 20, 2014 at 04:48:30PM +0100, David Herrmann wrote:
>> On Thu, Mar 20, 2014 at 4:32 PM,  <tytso at mit.edu> wrote:
>>> Why not make sealing an attribute of the "struct file", and enforce it
>>> at the VFS layer?  That way all file system objects would have access
>>> to sealing interface, and for memfd_shmem, you can't get another
>>> struct file pointing at the object, the security properties would be
>>> identical.
>>
>> Sealing as introduced here is an inode-attribute, not "struct file".
>> This is intentional. For instance, a gfx-client can get a read-only FD
>> via /proc/self/fd/ and pass it to the compositor so it can never
>> overwrite the contents (unless the compositor has write-access to the
>> inode itself, in which case it can just re-open it read-write).
> 
> Hmm, good point.  I had forgotten about the /proc/self/fd hole.
> Hmm... what if we have a SEAL_PROC which forces the permissions of
> /proc/self/fd to be 000?

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.

Actually, that feature would solve a major pet peeve of mine, I think: I
want something like memfd that allows me to keep the thing read-write
but that whomever I pass the fd to can't change.  With this feature, I
could do:

fd_rw = memfd_create (or O_TMPFILE or whatever)
fd_ro = open(/proc/self/fd/fd_ro, O_RDONLY);
fcntl(fd_ro, F_RESTRICT, F_RESTRICT_REOPEN);

send fd_ro via SCM_RIGHTS.

To really make this work well, I also want to SEAL_SHRINK the inode so
that the receiver can verify that I'm not going to truncate the file out
from under it.

Bingo, fast and secure one-way IPC.

--Andy