linux-kernel - Re: SEV guest regression in 4.18

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Message-ID: <47b8f9dc-fb55-f14d-cb88-343810ad5763@amd.com>
Date:   Fri, 24 Aug 2018 13:47:10 -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 08/24/2018 10:50 AM, Paolo Bonzini wrote:
> 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?
> 

No.

If we create two mapping of same physical address and ensure that
accesses are cache line aligned and size then I think we will safe from
cache point of view.

I have not tried early remap from kvmclock_init() yet and not sure if it
will work so early. The downside of this approach is, we will put
too many constraints on caller; it will need to ensure that variables
are cache line aligned and sized, never accessed with C=1, and must
create a new mapping before accessing it.

I am more inclined towards creating a new section with PMD aligned and
sized. This section will contains the decrypted data. In early
boot code we will update the mapping with C=0. If caller wants to create
a shared variable then it can do so with:

static int foo __decrypted;

I will submit patch fir review.

thanks