linux-kernel - Re: [PATCH, RFC 44/62] x86/mm: Set KeyIDs in encrypted VMAs for MKTME

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Date:   Mon, 17 Jun 2019 11:10:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Alison Schofield <alison.schofield@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...capital.net>,
        David Howells <dhowells@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>, linux-mm@...ck.org,
        kvm@...r.kernel.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC 44/62] x86/mm: Set KeyIDs in encrypted VMAs for MKTME

On Fri, Jun 14, 2019 at 12:11:23PM -0700, Dave Hansen wrote:
> On 6/14/19 11:46 AM, Alison Schofield wrote:
> > On Fri, Jun 14, 2019 at 11:26:10AM -0700, Dave Hansen wrote:
> >> On 6/14/19 10:33 AM, Alison Schofield wrote:
> >>> Preserving the data across encryption key changes has not
> >>> been a requirement. I'm not clear if it was ever considered
> >>> and rejected. I believe that copying in order to preserve
> >>> the data was never considered.
> >>
> >> We could preserve the data pretty easily.  It's just annoying, though.
> >> Right now, our only KeyID conversions happen in the page allocator.  If
> >> we were to convert in-place, we'd need something along the lines of:
> >>
> >> 	1. Allocate a scratch page
> >> 	2. Unmap target page, or at least make it entirely read-only
> >> 	3. Copy plaintext into scratch page
> >> 	4. Do cache KeyID conversion of page being converted:
> >> 	   Flush caches, change page_ext metadata
> >> 	5. Copy plaintext back into target page from scratch area
> >> 	6. Re-establish PTEs with new KeyID
> > 
> > Seems like the 'Copy plaintext' steps might disappoint the user, as
> > much as the 'we don't preserve your data' design. Would users be happy
> > w the plain text steps ?
> 
> Well, it got to be plaintext because they wrote it to memory in
> plaintext in the first place, so it's kinda hard to disappoint them. :)
> 
> IMNHO, the *vast* majority of cases, folks will allocate memory and then
> put a secret in it.  They aren't going to *get* a secret in some
> mysterious fashion and then later decide they want to protect it.  In
> other words, the inability to convert it is pretty academic and not
> worth the complexity.

I'm not saying it is (required to preserve); but I do think it is
somewhat surprising to have an mprotect() call destroy content. It's
traditionally specified to not do that.