On Thu, May 30, 2019 at 11:01 AM Sean Christopherson
[off-list ref] wrote:

On Thu, May 30, 2019 at 09:14:10AM -0700, Andy Lutomirski wrote:

quoted

On Thu, May 30, 2019 at 8:04 AM Stephen Smalley [off-list ref] wrote:

quoted

On 5/30/19 10:31 AM, Andy Lutomirski wrote:

quoted

Hi all-

After an offline discussion with Sean yesterday, here are some updates
to the user API parts of my proposal.

Unfortunately, Sean convinced me that MAXPERM doesn't work the way I
described it because, for SGX2, the enclave loader won't know at load
time whether a given EAUG-ed page will ever be executed.  So here's an
update.

First, here are the requrements as I see them, where EXECUTE, EXECMOD,
and EXECMEM could be substituted with other rules at the LSM's
discretion:

  - You can create a WX or RWX mapping if and only if you have EXECMEM.

  - To create an X mapping of an enclave page that has ever been W, you
need EXECMOD.

EXECMOD to what file? The enclave file from which the page's content
originated, the sigstruct file, or /dev/sgx/enclave?

I leave that decision to you :)  The user should need permission to do
an execmod thing on an enclave, however that wants to be encoded.

But that decision dictates how the SGX API handles sigstruct.  If LSMs
want to associate EXECMOD with sigstruct, then SGX needs to take sigstruct
early and hold a reference to the file for the lifetime of the enclave.
And if we're going to do that, the whole approach of inheriting
permissions from source VMAs becomes unnecessary complexity.

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  - To create an X mapping of an enclave page that came from EADD, you
need EXECUTE on the source file.  Optionally, we could also permit
this if you have EXECMOD.

What is the "source file" i.e. the target of the check?  Enclave file,
sigstruct file, or /dev/sgx/enclave?

Enclave file -- that is, the file backing the vma from which the data is loaded.

It wasn't explicitly called out in Andy's proposal(s), but the idea is
that the SGX driver would effectively inherit permissions from the source
VMA (EADD needs a source for the initial value of the encave page).

I actually meant for it to *not* work like this.  I don't want the
source VMA to have to be VM_EXEC.  I think the LSM should just check
permissions on ->vm_file.