On Mon, Jun 10, 2019 at 11:29 AM Xing, Cedric <cedric.xing@xxxxxxxxx> wrote:
>
> > 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.
This seems like an odd workflow. Shouldn't the #PF return back to
untrusted userspace so that the untrusted user code can make its own
decision as to whether it wants to EAUG a page there as opposed to,
say, killing the enclave or waiting to keep resource usage under
control?
