linux-kernel - Re: SEV guest regression in 4.18

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Message-ID: <25d2cf1f-11ee-ecba-73a9-15a9553a3d96@redhat.com>
Date:   Fri, 24 Aug 2018 17:50:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Borislav Petkov <bp@...e.de>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: SEV guest regression in 4.18

On 24/08/2018 17:41, Brijesh Singh wrote:
>>>
>>> Wouldn't that result in exposing/leaking whatever code/data happened
>>> to reside on the same 2M page (or corrupting it if the entire page
>>> isn't decrypted)?  Or are you suggesting that we'd also leave the
>>> encrypted mapping intact?
>>
>> Yes, exactly the latter, because...
> 
> 
> Hardware does not enforce coherency between the encrypted and
> unencrypted mapping for the same physical page. So, creating a
> two mapping of same physical address will lead a possible data
> corruption.
> 
> Note, SME creates two mapping of the same physical address to perform
> in-place encryption of kernel and initrd images; this is a special case
> and APM documents steps on how to do this.

Ah, so that's what I was thinking about.  But a single cache line would
never be used both encrypted and unencrypted, would it?

Paolo