linux-kernel - Re: Partially Privileged Applications

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Date:	Mon, 11 Nov 2013 19:44:47 +0100
From:	Matthias Schniedermeyer <ms@...d.de>
To:	Shahbaz Youssefi <shabbyx@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Partially Privileged Applications

On 11.11.2013 14:05, Shahbaz Youssefi wrote:
> On Sun, Nov 10, 2013 at 8:06 PM, Matthias Schniedermeyer <ms@...d.de> wrote:
> > I don't see a way around "borders" (Papers please), otherwise you can't
> > reject things you don't want, you have to check if that something that
> > is to be done is allowed. For e.g. you would get around every
> > permission-check, because the code you called is allowed to do
> > everything.
> 
> You're right actually. Proper linking solves the issue for "good people",
> but I can't think of a not-dirty way for preventing bad calls from
> "bad people". I may get back here if I do find a solution.

After thinking a bit about this.

It depends on the samantics of when the processor switches privileges. 
IF the processor only ever switches privileges for jmp/call/ret(...) you 
could place a privileged page with nothing but NOPs before the function 
that you actually want to call with elevated privileges, but whose page 
is NOT marked as privileged.

You then call/jump to the last NOP and let the CPU "slide" over into the 
next page. This way you could be sure that the necessary check-part of 
the function can't be circumvented and still have the higher privileges. 
But then what about functions the function might have to call, they 
would need to be in a privileged page or you would need a "keep 
privileges, regardless of page attribute" jmp/call. In the case of that 
functions being in a privileged page, you would have to make sure that 
you can't call them directly as "bad people" would just do that.

You would then need this kind auf NOP-Page before each function you want 
to be able to call, which wastes a certain amount of memory and also 
forces you to align each function to the beginning of a page (which also 
wastes memory). But what is a megabyte or so of wasted memory with 
gigabytes of memory. :-)

But essentially it would still be similar to the mentioned call gates.

All in all it still sounds like a nightmare to "get right".

-- 

Matthias
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