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

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Apr 10, 2014 at 4:16 PM, David Herrmann <dh.herrmann@gmail.com> wrote:
> Hi
>
> On Fri, Apr 11, 2014 at 1:05 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>> /proc/pid/fd is a really weird corner case in which the mode of an
>> inode that doesn't have a name matters.  I suspect that almost no one
>> will ever want to open one of these things out of /proc/self/fd, and
>> those who do should be made to think about it.
>
> I'm arguing in the context of memfd, and there's no security leak if
> people get access to the underlying inode (at least I'm not aware of
> any).

I'm not sure what you mean.

> As I said, context information is attached to the inode, not
> file context, so I'm fine if people want to open multiple file
> contexts via /proc. If someone wants to forbid open(), I want to hear
> _why_. I assume the memfd object has uid==uid-of-creator and
> mode==(777 & ~umask) (which usually results in X00, so no access for
> non-owners). I cannot see how /proc is a security issue here.

On further reflection, my argument for 000 is crap.  As far as I can
see, the only time that the mode matters at all when playing with
/proc/pid/fd, and they only way to get a non-O_RDWR memfd is using
/proc/pid/fd, so I'll argue for 0600 instead.

Argument why 0600 is better than 0600 & ~umask: either callers don't
care because the inode mode simply doesn't matter or they're using
/proc/pid/fd to *reduce* permissions, in which case they'd probably
like to avoid having to play with umask or call fchmod.

Argument why 0600 is better than 0777 & ~umask: People /prod/pid/fd
are the only ones who care, in which case they probably prefer for the
permissions not be increased by other users if they give them a
reduced-permission fd.

Anyway, this is all mostly unimportant.  Some text in the man page is
probably sufficient, but I still think that 0600 is trivial to
implement and a little bit more friendly.

--Andy

>
> Thanks
> David



-- 
Andy Lutomirski
AMA Capital Management, LLC

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