> From: Christopherson, Sean J > Sent: Wednesday, June 12, 2019 3:03 PM > > > I think this model works quite well in an SGX1 world. The main thing > > that makes me uneasy about this model is that, in SGX2, it requires > > that an SGX2-compatible enclave loader must pre-declare to the kernel > > whether it intends for its dynamically allocated memory to be > > ALLOW_EXEC. If ALLOW_EXEC is set but not actually needed, it will > > still fail if DENY_X_IF_ALLOW_WRITE ends up being set. The other > > version below does not have this limitation. > > I'm not convinced this will be a meaningful limitation in practice, > though that's probably obvious from my RFCs :-). That being said, the > UAPI quirk is essentially a dealbreaker for multiple people, so let's > drop #1. > > I discussed the options with Cedric offline, and he is ok with option #2 > *if* the idea actually translates to acceptable code and doesn't present > problems for userspace and/or future SGX features. > > So, I'll work on an RFC series to implement #2 as described below. If > it works out, yay! If not, i.e. option #2 is fundamentally broken, I'll > shift my focus to Cedric's code (option #3). > > > > 2. Pre-check LSM permissions and dynamically track mappings to > enclave > > > pages, e.g. add an SGX mprotect() hook to restrict W->X and WX > > > based on the pre-checked permissions. > > > > > > Pros: Does not impact SGX UAPI, medium kernel complexity > > > Cons: Auditing is complex/weird, requires taking enclave- > specific > > > lock during mprotect() to query/update tracking. > > > > Here's how this looks in my mind. It's quite similar, except that > > ALLOW_READ, ALLOW_WRITE, and ALLOW_EXEC are replaced with a little > > state machine. > > > > EADD does not take any special flags. It calls this LSM hook: > > > > int security_enclave_load(struct vm_area_struct *source); > > > > This hook can return -EPERM. Otherwise it 0 or > > ALLOC_EXEC_IF_UNMODIFIED (i.e. 1). This hook enforces permissions (a) > and (b). > > > > The driver tracks a state for each page, and the possible states are: > > > > - CLEAN_MAYEXEC /* no W or X VMAs have existed, but X is okay */ > > - CLEAN_NOEXEC /* no W or X VMAs have existed, and X is not okay */ > > - CLEAN_EXEC /* no W VMA has existed, but an X VMA has existed */ > > - DIRTY /* a W VMA has existed */ > > > > The initial state for a page is CLEAN_MAYEXEC if the hook said > > ALLOW_EXEC_IF_UNMODIFIED and CLEAN_NOEXEC otherwise. > > > > The future EAUG does not call a hook at all and puts pages into the > > state CLEAN_NOEXEC. If SGX3 or later ever adds EAUG-but-don't-clear, > > it can call security_enclave_load() and add CLEAN_MAYEXEC pages if > appropriate. > > > > EINIT takes a sigstruct pointer. SGX calls a new hook: > > > > unsigned int security_enclave_init(struct sigstruct *sigstruct, > > struct vm_area_struct *source, unsigned int flags); > > > > This hook can return -EPERM. Otherwise it returns 0 or a combination > > of flags DENY_WX and DENY_X_DIRTY. The driver saves this value. > > These represent permissions (c) and (d). > > > > If we want to have a permission for "execute code supplied from > > outside the enclave that was not measured", we could have a flag like > > HAS_UNMEASURED_CLEAN_EXEC_PAGE that the LSM could consider. > > > > mmap() and mprotect() enforce the following rules: > > > > - If VM_EXEC is requested and (either the page is DIRTY or VM_WRITE > is > > requested) and DENY_X_DIRTY, then deny. > > > > - If VM_WRITE and VM_EXEC are both requested and DENY_WX, then deny. > > > > - If VM_WRITE is requested, we need to update the state. If it was > > CLEAN_EXEC, then we reject if DENY_X_DIRTY. Otherwise we change > the > > state to DIRTY. > > > > - If VM_EXEC is requested and the page is CLEAN_NOEXEC, then deny. > > > > mprotect() and mmap() do *not* call SGX-specific LSM hooks to ask for > > permission, although they can optionally call an LSM hook if they hit > > one of the -EPERM cases for auditing purposes. > > > > Before the SIGSTRUCT is provided to the driver, the driver acts as > > though DENY_X_DIRTY and DENY_WX are both set. I think we've been discussing 2 topics simultaneously, one is the state machine that accepts/rejects mmap/mprotect requests, while the other is where is the best place to put it. I think we have an agreement on the former, and IMO option #2 and #3 differ only in the latter. Option #2 keeps the state machine inside SGX subsystem, so it could reuse existing data structures for page tracking/locking to some extent. Sean may have smarter ideas, but it looks to me like the existing 'struct sgx_encl_page' tracks individual enclave pages while the FSM states apply to ranges. So in order *not* to test page by page in mmap/mprotect, I guess some new range oriented structures are still necessary. But I don't think it very important anyway. My major concern is more from the architecture/modularity perspective. Specifically, the state machine is defined by LSM but SGX does the state transitions. That's a brittle relationship that'd break easily if the state machine changes in future, or if different LSM modules want to define different FSMs (comprised of different set of states and/or triggers). After all, what's needed by the SGX subsystem is just the decision, not the FSM definition. I think we should take a closer look at this area once Sean's patch comes out.
