linux-kernel - Re: [PATCHv5 02/19] mm: Do not use zero page in encrypted pages

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Date:   Thu, 19 Jul 2018 10:16:06 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 02/19] mm: Do not use zero page in encrypted pages

On Wed, Jul 18, 2018 at 10:36:24AM -0700, Dave Hansen wrote:
> On 07/17/2018 04:20 AM, Kirill A. Shutemov wrote:
> > Zero page is not encrypted and putting it into encrypted VMA produces
> > garbage.
> > 
> > We can map zero page with KeyID-0 into an encrypted VMA, but this would
> > be violation security boundary between encryption domains.
> 
> Why?  How is it a violation?
> 
> It only matters if they write secrets.  They can't write secrets to the
> zero page.

I believe usage of zero page is wrong here. It would indirectly reveal
content of supposedly encrypted memory region.

I can see argument why it should be okay and I don't have very strong
opinion on this.

If folks see it's okay to use zero page in encrypted VMAs I can certainly
make it work.

> Is this only because you accidentally inherited ->vm_page_prot on the
> zero page PTE?

Yes, in previous patchset I mapped zero page with wrong KeyID. This is one
of possible fixes for this.

-- 
 Kirill A. Shutemov