Re: [intel-sgx-kernel-dev] [PATCH v11 13/13] intel_sgx: in-kernel launch enclave

From: Andy Lutomirski <luto@kernel.org>
To: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: X86 ML <x86@kernel.org>,
	Platform Driver <platform-driver-x86@vger.kernel.org>,
	nhorman@redhat.com, npmccallum@redhat.com,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	intel-sgx-kernel-dev@lists.01.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Darren Hart <dvhart@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	andy@infradead.org
Subject: Re: [intel-sgx-kernel-dev] [PATCH v11 13/13] intel_sgx: in-kernel launch enclave
Date: Sat, 9 Jun 2018 22:39:50 -0700	[thread overview]
Message-ID: <CALCETrW3aZJMMm1GqgPm-cOD3p7JvKxLj7j85KJwfNW2iHTj4A@mail.gmail.com> (raw)
In-Reply-To: <20180608171216.26521-14-jarkko.sakkinen@linux.intel.com>

On Fri, Jun 8, 2018 at 10:32 AM Jarkko Sakkinen
<jarkko.sakkinen@linux.intel.com> wrote:
>
> The Launch Enclave (LE) generates cryptographic launch tokens for user
> enclaves. A launch token is used by EINIT to check whether the enclave
> is authorized to launch or not. By having its own launch enclave, Linux
> has full control of the enclave launch process.
>
> LE is wrapped into a user space proxy program that reads enclave
> signatures outputs launch tokens. The kernel-side glue code is
> implemented by using the user space helper framework.  The IPC between
> the LE proxy program and kernel is handled with an anonymous inode.
>
> The commit also adds enclave signing tool that is used by kbuild to
> measure and sign the launch enclave. CONFIG_INTEL_SGX_SIGNING_KEY points
> to a PEM-file for the 3072-bit RSA key that is used as the LE public key
> pair. The default location is:
>
>   drivers/platform/x86/intel_sgx/sgx_signing_key.pem
>
> If the default key does not exist kbuild will generate a random key and
> place it to this location. KBUILD_SGX_SIGN_PIN can be used to specify
> the passphrase for the LE public key.

It seems to me that it might be more useful to just commit a key pair
into the kernel.  As far as I know, there is no security whatsoever
gained by keeping the private key private, so why not make
reproducible builds easier by simply fixing the key?

Also, this email is so long that gmail won't let me quote the relevant
code, but: what is the intended use case for supporting the mode where
the MSRs are locked but happen to contain the right value?  I could
see the case for bundling a key with the kernel and literally
hard-coding the acceptable MSR values (as in literal values in the
code, not even autogenerated hashes).  The only use case I've thought
of for the code as it stands is that $VENDOR could publish their LE
public key and some daft firmware vendor could get it into their head
that it would be a good idea to lock the MSRs to that value.  This
would add no security at all, but it would add a considerable about of
annoyance and loss of value, so I still tend to think that we
shouldn't support it.