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Subject: Re: [PATCH, RFC 45/62] mm: Add the encrypt_mprotect() system call
 for MKTME
From:   Kai Huang <kai.huang@linux.intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        X86 ML <x86@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
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        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
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        David Howells <dhowells@redhat.com>,
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        keyrings@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>
Date:   Tue, 18 Jun 2019 14:23:31 +1200
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On Mon, 2019-06-17 at 18:43 -0700, Andy Lutomirski wrote:
> On Mon, Jun 17, 2019 at 6:35 PM Kai Huang <kai.huang@linux.intel.com> wrote:
> > 
> > 
> > > > > 
> > > > > I'm having a hard time imagining that ever working -- wouldn't it blow
> > > > > up if someone did:
> > > > > 
> > > > > fd = open("/dev/anything987");
> > > > > ptr1 = mmap(fd);
> > > > > ptr2 = mmap(fd);
> > > > > sys_encrypt(ptr1);
> > > > > 
> > > > > So I think it really has to be:
> > > > > fd = open("/dev/anything987");
> > > > > ioctl(fd, ENCRYPT_ME);
> > > > > mmap(fd);
> > > > 
> > > > This requires "/dev/anything987" to support ENCRYPT_ME ioctl, right?
> > > > 
> > > > So to support NVDIMM (DAX), we need to add ENCRYPT_ME ioctl to DAX?
> > > 
> > > Yes and yes, or we do it with layers -- see below.
> > > 
> > > I don't see how we can credibly avoid this.  If we try to do MKTME
> > > behind the DAX driver's back, aren't we going to end up with cache
> > > coherence problems?
> > 
> > I am not sure whether I understand correctly but how is cache coherence problem related to
> > putting
> > MKTME concept to different layers? To make MKTME work with DAX/NVDIMM, I think no matter which
> > layer
> > MKTME concept resides, eventually we need to put keyID into PTE which maps to NVDIMM, and kernel
> > needs to manage cache coherence for NVDIMM just like for normal memory showed in this series?
> > 
> 
> I mean is that, to avoid cache coherence problems, something has to
> prevent user code from mapping the same page with two different key
> ids.  If the entire MKTME mechanism purely layers on top of DAX,
> something needs to prevent the underlying DAX device from being mapped
> at the same time as the MKTME-decrypted view.  This is obviously
> doable, but it's not automatic.

Assuming I am understanding the context correctly, yes from this perspective it seems having
sys_encrypt is annoying, and having ENCRYPT_ME should be better. But Dave said "nobody is going to
do what you suggest in the ptr1/ptr2 example"? 

Thanks,
-Kai