On Fri, Sep 25, 2020 at 10:18:28AM -0700, Dave Hansen wrote: > Thanks for the walkthrough. The thing that clicked for me seeing those > examples was how the earlier ioctl(ADD_PAGE) is "bound" to later > enforcement actions at enclave PTE creation time. > > On 9/24/20 5:00 PM, Sean Christopherson wrote: > > My concern is that if we merge this > > > > ioctl(sgx_fd, ENCLAVE_ADD_PAGE, SGX_PROT_READ | SGX_PROT_EXEC, ptr, size); > > > > without ->mprotect(), we can't actually enforce the declared protections. And > > if we drop the field altogether: > > > > ioctl(sgx_fd, ENCLAVE_ADD_PAGE, ptr, size); > > > > then we can't implement security_enclave_load(). > > To me, it's perfectly OK to have parts of the ABI which are unused. It > sure makes them harder to test if there are no actual users in the code, > but if it solves a real problem with the ABI, I'm fine with it. > > Let's see if I can put all the pieces together. > > Background: > > 1. SGX enclave pages are populated with data by copying data to them > from normal memory via: ioctl(sgx_fd, ENCLAVE_ADD_PAGE, src_ptr...); > 2. We want to be able to restrict those normal memory data sources. For > instance, before copying data to an executable enclave page, we might > ensure that the source is executable. > 3. Enclave page permissions are dynamic just like normal permissions and > can be adjusted at runtime with mprotect() (along with a > corresponding special instruction inside the enclave) > 4. The original data source may have have long since vanished at the > time when enclave page permission are established (mmap() or > mprotect()) > > Solution: > > The solution is to force enclaves creators to declare their intent up > front to ioctl(ENCLAVE_ADD_PAGE). This intent can me immediately > compared to the source data mapping (and rejected if necessary). It is > also stashed off and then later compared with enclave PTEs to ensure > that any future mmap()/mprotect() operations performed by the enclave > creator or the enclave itself are consistent with the earlier declared > permissions. > > Essentially, this means that whenever the kernel is asked to change an > enclave PTE, it needs to ensure the change is consistent with that > stashed intent. There is an existing vm_ops->mmap() hook which allows > SGX to do that for mmap(). However, there is no ->mprotect() hook. Add > a vm_ops->mprotect() hook so that mprotect() operations which are > inconsistent with any page's stashed intent can be rejected by the driver. Yes to all of the above. > Implications: > > However, there is currently no implementation of the intent checks at > the time of ioctl(ENCLAVE_ADD_PAGE). Correct. > That means that the intent argument (SGX_PROT_*) is currently unused. No, the intent argument is used (eventually) by SGX's ->mprotect() implementation, i.e. sgx_mprotect() enforces that the actual protections are a subset of the declared/intended protections. If ->mprotect() is not merged, then it yes, it will be unused. And therein lies the problem as the kernel can't start using/enforcing the intent without breaking userspace. E.g. an enclave loaded with SGX_PROT_READ but mprotect()'d with PROT_READ | PROT_EXEC would break if sgx_mprotect() came along. One way to avoid introducing ->mprotect() would be to require all enclaves to declare all pages with READ|WRITE|EXEC. Then we could drop sgx_mprotect() since the mprotect() permissions are guaranteed to be a subset of the declared permissions. That would have the added bonus of eliminating the per-page checks in sgx_mmap()/sgx_mprotect(), though I've no idea if that is a meaningful optmization or it's lost in the noise. The big downside of requiring READ|WRITE|EXEC is that it will make life hell for a LSM policy owner if they ever want to apply EXECMEM or EXECMOD style restritions on enclaves, i.e. if SELinux folks want to add security_enclave_load(). I find that I'm more or less ok with that approach, in no small part because introducing security_enclave_load() might be a pretty big "if", e.g. security folks may decide that they'd rather allow/deny enclaves based on the measurement or signer of the enclave and eschew per-page checks entirely. > -- > > Is that all correct? Did I miss anything?
