SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)

[Andy Lutomirski]

On Thu, May 30, 2019 at 11:01 AM Sean Christopherson
<sean.j.christopherson at intel.com> wrote:
>
> On Thu, May 30, 2019 at 09:14:10AM -0700, Andy Lutomirski wrote:
> > On Thu, May 30, 2019 at 8:04 AM Stephen Smalley <sds at tycho.nsa.gov> wrote:
> > >
> > > On 5/30/19 10:31 AM, Andy Lutomirski wrote:
> > > > 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.
>
> > >
> > > >   - 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.