linux-kernel - Re: [RFC v2-fix 1/1] x86/tdx: Make DMA pages shared

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Message-ID: <3f392477-12b9-ff16-8e32-b00b6593d82a@intel.com>
Date:   Tue, 18 May 2021 15:31:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Kai Huang <kai.huang@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC v2-fix 1/1] x86/tdx: Make DMA pages shared

On 5/18/21 3:12 PM, Kuppuswamy, Sathyanarayanan wrote:
> "TDX is similar. In TDX architecture, the private guest memory is 
> encrypted, which prevents anything other than guest from
> accessing/modifying it. So to communicate with I/O devices, we need
> to create decrypted mapping and make the pages shared."

That's actually even more wrong. :(

Check out "Machine Check Architecture Background" in the TDX
architecture spec.

Modification is totally permitted in the architecture.  A host can write
all day long to guest memory.  Depending on how you use the word,
"access" can also include writes.

TDX really just prevents guests from *consuming* the gunk that an
attacker might write.

Also, don't say "decrypted".  The memory is probably still TME-enabled
and probably encrypted on the DIMM.  It's still encrypted even if
shared, it's just using the TME key, not the TD key.