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Subject: Re: [PATCHv5 03/19] mm/ksm: Do not merge pages with different KeyIDs
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
References: <20180717112029.42378-1-kirill.shutemov@linux.intel.com>
 <20180717112029.42378-4-kirill.shutemov@linux.intel.com>
 <a6fc50f2-b0c2-32db-cbef-3de57d5e6b16@intel.com>
 <20180719073240.autom4g4cdm3jgd6@kshutemo-mobl1>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Kai Huang <kai.huang@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <3045c925-f5a8-ae68-8f77-4cddaf040f9f@intel.com>
Date:   Thu, 19 Jul 2018 07:02:34 -0700
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On 07/19/2018 12:32 AM, Kirill A. Shutemov wrote:
> On Wed, Jul 18, 2018 at 10:38:27AM -0700, Dave Hansen wrote:
>> On 07/17/2018 04:20 AM, Kirill A. Shutemov wrote:
>>> Pages encrypted with different encryption keys are not allowed to be
>>> merged by KSM. Otherwise it would cross security boundary.
>> Let's say I'm using plain AES (not AES-XTS).  I use the same key in two
>> keyid slots.  I map a page with the first keyid and another with the
>> other keyid.
>>
>> Won't they have the same cipertext?  Why shouldn't we KSM them?
> We compare plain text, not ciphertext. And for good reason.

What's the reason?  Probably good to talk about it for those playing
along at home.

> Comparing ciphertext would only make KSM successful for AES-ECB that
> doesn't dependent on physical address of the page.
> 
> MKTME only supports AES-XTS (no plans to support AES-ECB). It effectively
> disables KSM if we go with comparing ciphertext.

But what's the security boundary that is violated?  You are talking
about some practical concerns (KSM scanning inefficiency) which is a far
cry from being any kind of security issue.