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SubjectRE: [intel-sgx-kernel-dev] [PATCH v7 4/8] intel_sgx: driver for Intel Software Guard Extensions
Date
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> wrote:
> +static void sgx_ewb(struct sgx_encl *encl, struct sgx_encl_page *entry)
> +{
> + struct sgx_va_page *va_page;
> + unsigned int va_offset;
> + int ret;
> + int i;
> +
> + for (i = 0; i < 2; i++) {
> + va_page = list_first_entry(&encl->va_pages,
> + struct sgx_va_page, list);
> + va_offset = sgx_alloc_va_slot(va_page);
> + if (va_offset < PAGE_SIZE)
> + break;
> +
> + list_move_tail(&va_page->list, &encl->va_pages);
> + }

This is broken, there is no guarantee that the next VA page will have
a free slot. You have to walk over all VA pages to guarantee a slot
is found, e.g. this caused EWB and ELDU errors.

> +
> + ret = __sgx_ewb(encl, entry, va_page, va_offset);
> + if (ret == SGX_NOT_TRACKED) {
> + /* slow path, IPI needed */
> + sgx_flush_cpus(encl);
> + ret = __sgx_ewb(encl, entry, va_page, va_offset);
> + }
> +
> + if (ret) {
> + sgx_invalidate(encl, true);
> + if (ret > 0)
> + sgx_err(encl, "EWB returned %d, enclave invalidated\n",
> + ret);
> + }
> +
> + sgx_free_page(entry->epc_page, encl);
> + entry->desc |= va_offset;
> + entry->va_page = va_page;
> + entry->desc &= ~SGX_ENCL_PAGE_RESERVED;
> +}

[...]

> +
> + /* Legal race condition, page is already faulted. */
> + if (entry->list.next != LIST_POISON1) {

Querying list.next to determine if an encl_page is resident in the EPC
is ugly and unintuitive, and depending on list's internal state seems
dangerous. Why not use a flag in the encl_page, e.g. as in the patch
I submitted almost 8 months ago for combining epc_page and va_page into
a union? And, the encl's SGX_ENCL_SECS_EVICTED flag can be dropped if
a flag is added to indicate whether or not any encl_page is resident in
the EPC.

https://lists.01.org/pipermail/intel-sgx-kernel-dev/2017-April/000570.html

> + if (reserve)
> + entry->desc |= SGX_ENCL_PAGE_RESERVED;
> + goto out;
> + }
> +
> + epc_page = sgx_alloc_page(SGX_ALLOC_ATOMIC);
> + if (IS_ERR(epc_page)) {
> + rc = PTR_ERR(epc_page);
> + epc_page = NULL;
> + goto out;
> + }
> +
> + /* If SECS is evicted then reload it first */
> + if (encl->flags & SGX_ENCL_SECS_EVICTED) {
> + secs_epc_page = sgx_alloc_page(SGX_ALLOC_ATOMIC);
> + if (IS_ERR(secs_epc_page)) {
> + rc = PTR_ERR(secs_epc_page);
> + secs_epc_page = NULL;
> + goto out;
> + }
> +
> + rc = sgx_eldu(encl, &encl->secs, secs_epc_page, true);
> + if (rc)
> + goto out;
> +
> + encl->secs.epc_page = secs_epc_page;
> + encl->flags &= ~SGX_ENCL_SECS_EVICTED;
> +
> + /* Do not free */
> + secs_epc_page = NULL;
> + }
> +
> + rc = sgx_eldu(encl, entry, epc_page, false /* is_secs */);
> + if (rc)
> + goto out;
> +
> + /* Track the EPC page even if vm_insert_pfn fails; we need to ensure
> + * the EPC page is properly freed and we can't do EREMOVE right away
> + * because EREMOVE may fail due to an active cpu in the enclave. We
> + * can't call vm_insert_pfn before sgx_eldu because SKL signals #GP
> + * instead of #PF if the EPC page is invalid.
> + */
> + encl->secs_child_cnt++;
> +
> + entry->epc_page = epc_page;
> +
> + if (reserve)
> + entry->desc |= SGX_ENCL_PAGE_RESERVED;
> +
> + /* Do not free */
> + epc_page = NULL;
> + list_add_tail(&entry->list, &encl->load_list);
> +
> + rc = vm_insert_pfn(vma, addr, SGX_EPC_PFN(entry->epc_page));
> + if (rc) {
> + /* Kill the enclave if vm_insert_pfn fails; failure only occurs
> + * if there is a driver bug or an unrecoverable issue, e.g. OOM.
> + */
> + sgx_crit(encl, "vm_insert_pfn returned %d\n", rc);
> + sgx_invalidate(encl, true);
> + goto out;
> + }
> +
> + sgx_test_and_clear_young(entry, encl);
> +out:
> + mutex_unlock(&encl->lock);
> + if (epc_page)
> + sgx_free_page(epc_page, encl);
> + if (secs_epc_page)
> + sgx_free_page(secs_epc_page, encl);
> + return rc ? ERR_PTR(rc) : entry;
> +}
>

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