RE: [RFC PATCH v2 2/5] x86/sgx: Require userspace to define enclave pages' protection bits

[Xing, Cedric]

> 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;
> +}
> +