linux-kernel - Re: SEV guest regression in 4.18

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Message-ID: <c565ae1c-c2f0-8a43-b1f5-404c2efe7c56@amd.com>
Date:   Fri, 24 Aug 2018 10:41:27 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     brijesh.singh@....com, 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 08/23/2018 11:16 AM, Paolo Bonzini wrote:
> On 23/08/2018 17:29, Sean Christopherson wrote:
>> On Thu, Aug 23, 2018 at 01:26:55PM +0200, Paolo Bonzini wrote:
>>> On 22/08/2018 22:11, Brijesh Singh wrote:
>>>>
>>>> Yes, this is one of approach I have in mind. It will avoid splitting
>>>> the larger pages; I am thinking that early in boot code we can lookup
>>>> for this special section and decrypt it in-place and probably maps with
>>>> C=0. Only downside, it will increase data section footprint a bit
>>>> because we need to align this section to PM_SIZE.
>>>
>>> If you can ensure it doesn't span a PMD, maybe it does not need to be
>>> aligned; you could establish a C=0 mapping of the whole 2M around it.
>>
>> 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.


> 
>> Does hardware include the C-bit in the cache tag?
> 
> ... the C-bit is effectively part of the physical address and hence of
> the cache tag.  The kernel is already relying on this to properly
> encrypt/decrypt pages, if I remember correctly.
> 
> Paolo
>