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SubjectRe: [PATCHv5 03/19] mm/ksm: Do not merge pages with different KeyIDs
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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.

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