linux-kernel - Re: [RFC PATCH] mm: extend memfd with ability to create "secret" memory areas

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Date:   Wed, 26 Aug 2020 09:54:57 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alan Cox <alan@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christopher Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [RFC PATCH] mm: extend memfd with ability to create "secret"
 memory areas

On Fri, Aug 14, 2020 at 11:09 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 8/14/20 10:46 AM, Andy Lutomirski wrote:
> > I'm a little unconvinced about the security benefits.  As far as I
> > know, UC memory will not end up in cache by any means (unless
> > aliased), but it's going to be tough to do much with UC data with
> > anything resembling reasonable performance without derived values
> > getting cached.
>
> I think this is much more in the category of raising the bar than
> providing any absolute security guarantees.

The problem here is that we're raising the bar in a way that is
weirdly architecture dependent, *extremely* nonperformant, and may not
even accomplish what it's trying to accomplish.

>
> Let's say you have a secret and you read it into some registers and then
> spill them on the stack.  You've got two cached copies, one for the
> primary data and another for the stack copy.  Secret areas don't get rid
> of the stack copy, but they do get rid of the other one.  One cache copy
> is better than two.  Bar raised. :)

If we have two bars right next to each other and we raise one of them,
did we really accomplish much?  I admit that having a secret in its
own dedicated cache line seems like an easier target than a secret in
a cache line that may be quickly overwritten by something else.  But
even user registers right now aren't specially protected -- pt_regs
lives is cached and probably has a predictable location, especially if
you execve() a setuid program.

>
> There are also some stronger protections, less in the bar-raising
> category.  On x86 at least, uncached accesses also crush speculation.
> You can't, for instance, speculatively get wrong values if you're not
> speculating in the first place.  I was thinking of things like Load
> Value Injection[1].

This seems genuinely useful, but it doesn't really address the fact
that requesting UC memory via PAT apparently has a good chance of
getting WB anyway.

>
> I _believe_ there are also things like AES-NI that can get strong
> protection from stuff like this.  They load encryption keys into (AVX)
> registers and then can do encrypt/decrypt operations without the keys
> leaving the registers.  If the key was loaded from a secret memory area
> right into the registers, I think the protection from cache attacks
> would be pretty strong.
>

Except for context switches :)
>
> 1.
> https://software.intel.com/security-software-guidance/insights/deep-dive-load-value-injection