On Wed, Jun 12, 2019 at 12:30:20PM -0700, Andy Lutomirski wrote: > On Tue, Jun 11, 2019 at 3:02 PM Sean Christopherson > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > 1. Require userspace to explicitly specificy (maximal) enclave page > > permissions at build time. The enclave page permissions are provided > > to, and checked by, LSMs at enclave build time. > > > > Pros: Low-complexity kernel implementation, straightforward auditing > > Cons: Sullies the SGX UAPI to some extent, may increase complexity of > > SGX2 enclave loaders. > > In my notes, this works like this. This is similar, but not > identical, to what Sean has been sending out. ... > mmap() and mprotect() enforce the following rules: > > - Deny if a PROT_ flag is requested but the corresponding ALLOW_ flag > is not set for all pages in question. > > - Deny if PROT_WRITE, PROT_EXEC, and DENY_WX are all set. > > - Deny if PROT_EXEC, ALLOW_WRITE, and DENY_X_IF_ALLOW_WRITE are all set. > > 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. IMO, #1 only makes sense if it's stripped down to avoid auditing and locking complications, i.e. gets a pass/fail at security_enclave_load() and clears VM_MAY* flags during mmap(). If we want WX and W->X to be differentiated by security_enclave_init() as opposed to security_enclave_load(), then we should just scrap #1. > 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.
