linux-kernel - Re: SEV guest regression in 4.18

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Date:   Fri, 24 Aug 2018 13:48:52 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     brijesh.singh@....com, Paolo Bonzini <pbonzini@...hat.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 08/24/2018 11:24 AM, Sean Christopherson wrote:
> On Fri, Aug 24, 2018 at 10:41:27AM -0500, Brijesh Singh wrote:
>>
>>
>> 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.
> 
> But couldn't we avoid corruption by ensuring data accessed via the
> unencrypted mapping is cache line aligned and sized?  The CPU could
> speculatively bring the encrypted version into the cache but it
> should never get into a modified state (barring a software bug, but
> that would be a problem regardless of encryption).
> 

Yes, if we can ensure that accessed are cache line aligned and sized
then we should be fine.