linux-kernel - Re: [PATCH, RFC 03/62] mm/ksm: Do not merge pages with different KeyIDs

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Message-ID: <1697adad-6ae2-ea85-bab5-0144929ed2d9@intel.com>
Date:   Fri, 10 May 2019 11:07:11 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "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>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        David Howells <dhowells@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Alison Schofield <alison.schofield@...el.com>,
        linux-mm@...ck.org, kvm@...r.kernel.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC 03/62] mm/ksm: Do not merge pages with different
 KeyIDs

On 5/8/19 7:43 AM, Kirill A. Shutemov wrote:
> KeyID indicates what key to use to encrypt and decrypt page's content.
> Depending on the implementation a cipher text may be tied to physical
> address of the page. It means that pages with an identical plain text
> would appear different if KSM would look at a cipher text. It effectively
> disables KSM for encrypted pages.
> 
> In addition, some implementations may not allow to read cipher text at all.
> 
> KSM compares plain text instead (transparently to KSM code).
> 
> But we still need to make sure that pages with identical plain text will
> not be merged together if they are encrypted with different keys.
> 
> To make it work kernel only allows merging pages with the same KeyID.
> The approach guarantees that the merged page can be read by all users.

I can't really parse this description.  Can I suggest replacement text?

Problem: KSM compares plain text.  It might try to merge two pages that
have the same plain text but different ciphertext and possibly different
encryption keys.  When the kernel encrypted the page, it promised that
it would keep it encrypted with _that_ key.  That makes it impossible to
merge two pages encrypted with different keys.

Solution: Never merge encrypted pages with different KeyIDs.