Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

Jarkko Sakkinen
2020-09-28T16:20:10Z

On Mon, Sep 28, 2020 at 07:04:38AM -0700, Dave Hansen wrote:
> On 9/27/20 5:53 PM, Jarkko Sakkinen wrote:
> > On Fri, Sep 25, 2020 at 12:53:35PM -0700, Dave Hansen wrote:
> >> On 9/25/20 12:43 PM, Sean Christopherson wrote:
> >>>> That means that the intent argument (SGX_PROT_*) is currently unused.
> >>> No, the intent argument is used (eventually) by SGX's ->mprotect()
> >>> implementation, i.e. sgx_mprotect() enforces that the actual protections are a
> >>> subset of the declared/intended protections.
> >>>
> >>> If ->mprotect() is not merged, then it yes, it will be unused.
> >>
> >> OK, I think I've got it.
> >>
> >> I think I'm OK with adding ->mprotect(). As long as folks buy into the
> >> argument that intent needs to be checked at mmap() time, they obviously
> >> need to be checked at mprotect() too.
> >>
> >> Jarkko, if you want to try and rewrite the changelog, capturing the
> >> discussion here and reply, I think I can ack the resulting patch. I
> >> don't know if that will satisfy the request from Boris from an ack from
> >> a "mm person", but we can at least start there. :)
> >
> > I think what it needs, based on what I've read, is the step by step
> > description of the EMODPE scenarion without this callback and with it.
>
> EMODPE is virtually irrelevant for this whole thing. The x86 PTE
> permissions still specify the most restrictive permissions, which is
> what matters the most.
>
> We care about the _worst_ the enclave can do, not what it imposes on
> itself on top of that.

AFAIK it is not, or what we are protecting against with this anyway
then?

Let say an LSM makes decision for the permissions based on origin. If we
do not have this you can:

1. EMODPE
2. mprotect

I.e. whatever LSM decides, won't matter.

The other case, noexec, is now unconditionally denied.

> > I think other important thing to underline is that an LSM or any other
> > security measure can only do a sane decision when the enclave is loaded.
> > At that point we know the source (vm_file).
>
> Right, you know the source, but it can be anonymous or a file.

They are both origin, the point being that you know what you're dealing
with when you build the enclave, not when you map it.

This is my current rewrite of the commit message in my master branch:

"
mm: Add 'mprotect' callback to vm_ops

Intel Sofware Guard eXtensions (SGX) allows creation of blobs called
enclaves, for which page permissions are defined when the enclave is first
loaded. Once an enclave is loaded and initialized, it can be mapped to the
process address space.

There is no standard file format for enclaves. They are dynamically built
and the ways how enclaves are deployed differ greatly. For an app you might
want to have a simple static binary, but on the other hand for a container
you might want to dynamically create the whole thing at run-time. Also, the
existing ecosystem for SGX is already large, which would make the task very
hard.

Finally, even if there was a standard format, one would still want a
dynamic way to add pages to the enclave. One big reason for this is that
enclaves have load time defined pages that represent entry points to the
enclave. Each entry point can service one hardware thread at a time and
you might want to run-time parametrize this depending on your environment.

The consequence is that enclaves are best created with an ioctl API and the
access control can be based only to the origin of the source file for the
enclave data, i.e. on VMA file pointer and page permissions. For example,
this could be done with LSM hooks that are triggered in the appropriate
ioctl's and they could make the access control decision based on this
information.

Unfortunately, there is ENCLS[EMODPE] that a running enclave can use to
upgrade its permissions. If we do not limit mmap() and mprotect(), enclave
could upgrade its permissions by using EMODPE followed by an appropriate
mprotect() call. This would be completely hidden from the kernel.

Add 'mprotect' hook to vm_ops, so that a callback can be implemeted for SGX
that will ensure that {mmap, mprotect}() permissions do not surpass any of
the original page permissions. This feature allows to maintain and refine
sane access control for enclaves.
"

I'm mostly happy with this but am open for change suggestions.

/Jarkko