On Fri, May 24, 2019 at 01:42:13PM -0700, Xing, Cedric wrote: > > From: linux-sgx-owner@xxxxxxxxxxxxxxx [mailto:linux-sgx- > > owner@xxxxxxxxxxxxxxx] On Behalf Of Sean Christopherson > > Sent: Friday, May 24, 2019 12:14 PM > > > > My point is that enclaves have different properties than shared objects. > > > > Normal LSM behavior with regard to executing files is to label files > > with e.g. FILE__EXECUTE. Because an enclave must be built to the exact > > specifications of .sigstruct, requring FILE__EXECUTE on the .sigstruct > > is effectively the same as requiring FILE__EXECUTE on the enclave itself. > > > > Addressing your scenario of loading an executable page in EPC, doing so > > would require one of the following: > > > > - Ability to install a .sigstruct with FILE__EXECUTE > > > > - PROCESS__EXECMEM > > > > - FILE__EXECMOD and SGX2 support > > Now I got your point. It sounds a great idea to me! > > But instead of using .sigstruct file, I'd still recommend using file mapping > (i.e. SIGSTRUCT needs to reside in executable memory). But then there'll be a Why? Even in the Graphene case the final .sigstruct can be known ahead of time. Userspace can always use memfd() if it's generating SIGSTRUCT on the fly. > hole - a process having FILE__EXECMOD on any file could use that file as a > SIGSTRUCT. Probably we'll need a new type in SELinux to label > enclave/sigstruct files. > > > I don't see a fundamental difference between having RWX in an enclave > > and RWX in normal memory, either way the process can execute arbitrary > > code, i.e. PROCESS__EXECMEM is appropriate. Yes, an enclave will #UD on > > certain instructions, but that's easily sidestepped by having a > > trampoline in the host (marked RX) and piping arbitrary code into the > > enclave. Or using EEXIT to do a bit of ROP. > > I'm with you. > > With your proposal only FILE__EXECMOD is needed on /dev/sgx/enclave to launch > Graphene enclaves or the like. It wouldn't even need FILE__EXECMOD, assuming Graphene does all of its libc rewriting before building the enclave, i.e. doesn't EADD RWX pages. > > > > > No changes are required to LSMs, SGX1 has a single LSM touchpoint > > > > > in > > > > its > > > > > mmap(), and I *think* the only required userspace change is to > > > > > mmap() PROT_NONE when allocating the enclave's virtual address > > range. > > > > > > I'm not sure I understand the motivation behind this proposal to > > > decouple initial EPC permissions from source pages. > > > > Pulling permissions from source pages means userspace needs to fully map > > the in normal memory, including marking pages executable. That exposes > > the loader to having executable pages in its address space that it has > > no intention of executing (outside of the enclave). And for Graphene, > > it means having to actively avoid PROCESS__EXECMEM, e.g. by using a > > dummy backing file to build the enclave instead of anon memory. > > Agreed. > > > > > > I don't think it a big deal to fully mmap() enclave files, which have > > > to be parsed by user mode anyway to determine various things including > > > but not limited to the size of heap(s), size and number of > > > TCSs/stacks/TLS areas, and the overall enclave size. So with PHDRs > > > parsed, it's trivial to mmap() each segment with permissions from its > > PHDR. > > > > > > > > As for Graphene, it doesn't need extra permissions to run its > > > > > enclaves, it just needs a way to install .sigstruct, which is a > > > > > generic permissions problem and not SGX specific. > > > > > > > > > > > > > > > For SGX2 maybe: > > > > > > > > > > - No additional requirements to map an EAUG'd page as RW page. > > Not > > > > > aligned with standard MAP_SHARED behavior, but we really don't > > want > > > > > to require FILE__WRITE, and thus allow writes to .sigstruct. > > > > > > > > > > - Require FILE__EXECMOD on the .sigstruct to map previously > > writable > > > > > page as executable (which indirectly includes all EAUG'd > > pages). > > > > > Wiring this up will be a little funky, but we again we don't > > want > > > > > to require FILE__WRITE on .sigstruct. > > > > > > > > > > > I'm lost. Why is EAUG tied to permissions on .sigstruct? > > > > Because for the purposes of LSM checks, .sigstruct is the enclave's > > backing file, and mapping a previously writable enclave page as > > exectuable is roughly equivalent to mapping a CoW'd page as exectuable. > > I think I've got your idea. You are trying to use permissions on .sigstruct > to determine whether EAUG will be available to that specific enclave. Am I > right? Yep. > I'd tie EAUG to the permissions of /dev/sgx/enclave instead. But why? There > are couple of reasons. For one, a SIGSTRUCT identifies the behavior of the > enclave, hence the SGX features needed by that enclave. So if an enclave > requires EAUG, the .sigstruct has to allow EAUG or the enclave wouldn't work. > That means the system admin wouldn't have a choice but to match up what's > needed by the enclave. For two, whether to allow, say loading code > dynamically into an enclave, depends on whether the host process can tolerate > the inherent risk. And that decision is seldom made on individual enclaves > but to the host process as a whole. And /dev/sgx/enclave serves that purpose. I think I'd be ok either way? What I really care about is having line of sight to a sane way to support for SGX2, and both seem sane. I.e. we can hash this detail out when SGX2 goes in.
