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

From: Andy Lutomirski
Date: Tue May 14 2019 - 16:47:52 EST

> On May 14, 2019, at 8:30 AM, Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx> wrote:
>> 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.

As an example, SELinux has an âexecuteâ permission (via
security_mmap_file â see file_map_prot_check()) that controls whether
you can execute code from that file. If you lack this permission on a
file, you may still be able to map it PROT_READ, but you may not map
it PROT_EXEC. Similarly, if you want to malloc() some memory, write
*code* to it, and execute it, you need a specific permission.

So, unless we somehow think itâs okay for SGX to break the existing
model, we need to respect these restrictions in the SGX driver. In
other words, we either need to respect execmem, etc or require
PROT_EXEC or the equivalent. I like the latter a lot more.