linux-kernel - RE: [intel-sgx-kernel-dev] [PATCH v7 4/8] intel_sgx: driver for Intel Software Guard Extensions

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Date:   Thu, 7 Dec 2017 14:46:39 +0000
From:   "Christopherson, Sean J" <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        "intel-sgx-kernel-dev@...ts.01.org" 
        <intel-sgx-kernel-dev@...ts.01.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Shevchenko <andy@...radead.org>
Subject: RE: [intel-sgx-kernel-dev] [PATCH v7 4/8] intel_sgx: driver for
 Intel Software Guard Extensions

Jarkko Sakkinen <jarkko.sakkinen@...ux.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;
> +}
>