Re: [PATCH v20 00/28] Intel SGX1 support

From: Haitao Huang
Date: Tue May 14 2019 - 11:32:16 EST

On Tue, 14 May 2019 10:17:29 -0500, Andy Lutomirski <luto@xxxxxxxxxx> wrote:

On Tue, May 14, 2019 at 7:33 AM Haitao Huang
<haitao.huang@xxxxxxxxxxxxxxx> wrote:

On Fri, 10 May 2019 14:22:34 -0500, Andy Lutomirski <luto@xxxxxxxxxx>
wrote:

> On Fri, May 10, 2019 at 12:04 PM Jethro Beekman <jethro@xxxxxxxxxxxx>
> wrote:
>>
>> On 2019-05-10 11:56, Xing, Cedric wrote:
>> > Hi Jethro,
>> >
>> >> ELF files are explicitly designed such that you can map them (with
>> mmap)
>> >> in 4096-byte chunks. However, sometimes there's overlap and you will
>> >> sometimes see that a particular offset is mapped twice because the
>> first
>> >> half of the page in the file belongs to an RX range and the second
>> half
>> >> to an R-only range. Also, ELF files don't (normally) describe stack,
>> >> heap, etc. which you do need for enclaves.
>> >
>> > You have probably misread my email. By mmap(), I meant the enclave
>> file would be mapped via *multiple* mmap() calls, in the same way as
>> what dlopen() would do in loading regular shared object. The intention
>> here is to make the enclave file subject to the same checks as regular
>> shared objects.
>>
>> No, I didn't misread your email. My original point still stands:
>> requiring that an enclave's memory is created from one or more mmap
>> calls of a file puts significant restrictions on the enclave's on-disk
>> representation.
>>
>
> For a tiny bit of background, Linux (AFAIK*) makes no effort to ensure
> the complete integrity of DSOs. What Linux *does* do (if so
> configured) is to make sure that only approved data is mapped
> executable. So, if you want to have some bytes be executable, those
> bytes have to come from a file that passes the relevant LSM and IMA
> checks.

Given this, I just want to step back a little to understand the exact
issue that SGX is causing here for LSM/IMA. Sorry if I missed points
discussed earlier.

By the time of EADD, enclave file is opened and should have passed IMA and
SELinux policy enforcement gates if any. We really don't need extra mmaps
on the enclave files to be IMA and SELinux compliant.

The problem, as i see it, is that they passed the *wrong* checks,
because, as you noticed:

We are loading
enclave files as RO and copying those into EPC.

Which is, semantically, a lot like loading a normal file as RO and
then mprotecting() it to RX, which is disallowed under quite a few LSM
policies.

An IMA policy can enforce
RO files (or any file). And SELinux policy can say which processes can
open the file for what permissions. No extra needed here.

If SELinux says a process may open a file as RO, that does *not* mean
that it can be opened as RX.


But in this case, file itself is mapped as RO treated like data and it is not for execution. SGX enclave pages have EPCM enforced permissions. So from SELinux point of view I would think it can treat it as RO and that's fine.


And sgx enclaves are always signed and integrity protected and verified at
the time of EINIT. So if EINIT passes, we know the content loaded
(including permission flags) is matching the sigstruct. But
sigstruct/signature is part of the file, should be accounted for in IMA
measurement of the whole file, so it is also verified by IMA during file
open, right?

This does work, but only if the kernel parses that file so that the
kernel can trust that the enclave data actually came from the file as
intended. And moving the parsing to the kernel seems like a mess that
no one really wants to do.

If kernel only needs to know the source bytes are from a file that passed IMA integrity and SELinux RO enforcement, then it can just check if the source pointer belongs to a VMA with valid fd and no parsing or checking permissions needed.
Understood if you want to make enclave file code segment stick to the RX semantics mentioned above, then this doesn't qualify.

Thanks

Haitao