> From: Christopherson, Sean J
> Sent: Wednesday, June 05, 2019 7:12 PM
>
> +/**
> + * sgx_map_allowed - check vma protections against the associated
> enclave page
> + * @encl: an enclave
> + * @start: start address of the mapping (inclusive)
> + * @end: end address of the mapping (exclusive)
> + * @prot: protection bits of the mapping
> + *
> + * Verify a userspace mapping to an enclave page would not violate the
> +security
> + * requirements of the *kernel*. Note, this is in no way related to
> +the
> + * page protections enforced by hardware via the EPCM. The EPCM
> +protections
> + * can be directly extended by the enclave, i.e. cannot be relied upon
> +by the
> + * kernel for security guarantees of any kind.
> + *
> + * Return:
> + * 0 on success,
> + * -EACCES if the mapping is disallowed
> + */
> +int sgx_map_allowed(struct sgx_encl *encl, unsigned long start,
> + unsigned long end, unsigned long prot) {
> + struct sgx_encl_page *page;
> + unsigned long addr;
> +
> + prot &= (VM_READ | VM_WRITE | VM_EXEC);
> + if (!prot || !encl)
> + return 0;
> +
> + mutex_lock(&encl->lock);
> +
> + for (addr = start; addr < end; addr += PAGE_SIZE) {
> + page = radix_tree_lookup(&encl->page_tree, addr >>
> PAGE_SHIFT);
> +
> + /*
> + * Do not allow R|W|X to a non-existent page, or protections
> + * beyond those of the existing enclave page.
> + */
> + if (!page || (prot & ~page->prot))
> + return -EACCES;
In SGX2, pages will be "mapped" before being populated.
Here's a brief summary for those who don't have enough background on how new EPC pages could be added to a running enclave in SGX2:
- There are 2 new instructions - EACCEPT and EAUG.
- EAUG is used by SGX module to add (augment) a new page to an existing enclave. The newly added page is *inaccessible* until the enclave *accepts* it.
- EACCEPT is the instruction for an enclave to accept a new page.
And the s/w flow for an enclave to request new EPC pages is expected to be something like the following:
- The enclave issues EACCEPT at the linear address that it would like a new page.
- EACCEPT results in #PF, as there's no page at the linear address above.
- SGX module is notified about the #PF, in form of its vma->vm_ops->fault() being called by kernel.
- SGX module EAUGs a new EPC page at the fault address, and resumes the enclave.
- EACCEPT is reattempted, and succeeds at this time.
But with the above check in sgx_map_allowed(), I'm not sure how this will work out with SGX2.
> + }
> +
> + mutex_unlock(&encl->lock);
> +
> + return 0;
> +}
> +
