From patchwork Fri Jan 19 00:01:41 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Dan Williams X-Patchwork-Id: 125060 Delivered-To: patch@linaro.org Received: by 10.46.66.141 with SMTP id h13csp53248ljf; Thu, 18 Jan 2018 16:11:13 -0800 (PST) X-Google-Smtp-Source: ACJfBost8/xIJrJnGjrfD0QE4tTFmNHBFZEUUYJw6OiALpRTJdyCuUtlFDH6IFlQd7Zzk48TX1K1 X-Received: by 2002:a17:902:36a:: with SMTP id 97-v6mr634657pld.365.1516320673535; Thu, 18 Jan 2018 16:11:13 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1516320673; cv=none; d=google.com; s=arc-20160816; b=XswHojN2yZqb21ecTmPFiKMXqgN2hhY5S65d1P2QGWJkcfL06dne3MOH9R6Pg16vrA +ruCrpB7hvIN28X79S8twAFHgTn/cjLDeGd+1oox7Ii/Qofw+JKG8JldBHAV02Vf3646 5n2oyzx8a4hMRMRmL2IAtTlrrrgXYBjSnTZmTbvMFYyWpoJWBHWyOZ9wAHW1+63FIA1d hlHz155p4mKIt7LHDVmvAmfwiD5sAFO/99fBlGYwq58P7dIGw4neCaZ0ykn1xeoDbrxG +HYPSPHichbjNknZgejmEHKmzMAE8p/a42tKEAsmMi5JI+pw9IXirLaVjwzndwRFZ6qm sPLQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:content-transfer-encoding:mime-version :user-agent:references:in-reply-to:message-id:date:cc:to:from :subject:arc-authentication-results; bh=+CooYn4aNXo6cXY7BajUtDDyIICrD8ZQqtKRxShlTL8=; b=LJCvCA8pNyIpheQwt4b45fhpko7wC7b+R0EsmGJL/3GVzCx2omKy5BXs9jsa6cL6Sd XfEunU0E6MPReIwbf9/yoqPJcQyEUjykGN0Nl4lKFUB7ftFWmvY3924b5sE78W3s68J8 Zu1ov/Jx4SqUENX8L7/G5AnlaOGYF0G3yVK3cs8Q1eRMzAXb/dLPNT6YVTyTiQ0QHb7i veRF18Zd7aV6aOvKEpmp59/1UylQdSLS5mpLSbqN7Y2L8Ha6PQ30EDk9VCAvKIc5CYEK tIml/HHuta5ieECTjuHsjHatMbwjDOcY6E7QczdBmryhXfyPqNCGUy6ax9RkEvg2/rMb cziA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id y4si6875424pgv.74.2018.01.18.16.11.13; Thu, 18 Jan 2018 16:11:13 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755189AbeASALJ (ORCPT + 28 others); Thu, 18 Jan 2018 19:11:09 -0500 Received: from mga11.intel.com ([192.55.52.93]:4974 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754930AbeASAKs (ORCPT ); Thu, 18 Jan 2018 19:10:48 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Jan 2018 16:10:47 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,379,1511856000"; d="scan'208";a="27657720" Received: from dwillia2-desk3.jf.intel.com (HELO dwillia2-desk3.amr.corp.intel.com) ([10.54.39.16]) by orsmga002.jf.intel.com with ESMTP; 18 Jan 2018 16:10:47 -0800 Subject: [PATCH v4 01/10] Documentation: document array_ptr From: Dan Williams To: linux-kernel@vger.kernel.org Cc: Mark Rutland , linux-arch@vger.kernel.org, Kees Cook , kernel-hardening@lists.openwall.com, Peter Zijlstra , gregkh@linuxfoundation.org, Jonathan Corbet , Will Deacon , tglx@linutronix.de, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@linux.intel.com Date: Thu, 18 Jan 2018 16:01:41 -0800 Message-ID: <151632010172.21271.13545507895393426065.stgit@dwillia2-desk3.amr.corp.intel.com> In-Reply-To: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> References: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> User-Agent: StGit/0.17.1-9-g687f MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Mark Rutland Document the rationale and usage of the new array_ptr() helper. Signed-off-by: Mark Rutland Signed-off-by: Will Deacon Cc: Dan Williams Cc: Jonathan Corbet Cc: Peter Zijlstra Reviewed-by: Kees Cook Signed-off-by: Dan Williams --- Documentation/speculation.txt | 143 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 Documentation/speculation.txt diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..a47fbffe0dab --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,143 @@ +This document explains potential effects of speculation, and how undesirable +effects can be mitigated portably using common APIs. + +=========== +Speculation +=========== + +To improve performance and minimize average latencies, many contemporary CPUs +employ speculative execution techniques such as branch prediction, performing +work which may be discarded at a later stage. + +Typically speculative execution cannot be observed from architectural state, +such as the contents of registers. However, in some cases it is possible to +observe its impact on microarchitectural state, such as the presence or +absence of data in caches. Such state may form side-channels which can be +observed to extract secret information. + +For example, in the presence of branch prediction, it is possible for bounds +checks to be ignored by code which is speculatively executed. Consider the +following code: + + int load_array(int *array, unsigned int idx) + { + if (idx >= MAX_ARRAY_ELEMS) + return 0; + else + return array[idx]; + } + +Which, on arm64, may be compiled to an assembly sequence such as: + + CMP , #MAX_ARRAY_ELEMS + B.LT less + MOV , #0 + RET + less: + LDR , [, ] + RET + +It is possible that a CPU mis-predicts the conditional branch, and +speculatively loads array[idx], even if idx >= MAX_ARRAY_ELEMS. This value +will subsequently be discarded, but the speculated load may affect +microarchitectural state which can be subsequently measured. + +More complex sequences involving multiple dependent memory accesses may result +in sensitive information being leaked. Consider the following code, building +on the prior example: + + int load_dependent_arrays(int *arr1, int *arr2, int idx) + { + int val1, val2, + + val1 = load_array(arr1, idx); + val2 = load_array(arr2, val1); + + return val2; + } + +Under speculation, the first call to load_array() may return the value of an +out-of-bounds address, while the second call will influence microarchitectural +state dependent on this value. This may provide an arbitrary read primitive. + +==================================== +Mitigating speculation side-channels +==================================== + +The kernel provides a generic API to ensure that bounds checks are respected +even under speculation. Architectures which are affected by speculation-based +side-channels are expected to implement these primitives. + +The array_ptr() helper in can be used to prevent +information from being leaked via side-channels. + +A call to array_ptr(arr, idx, sz) returns a sanitized pointer to +arr[idx] only if idx falls in the [0, sz) interval. When idx < 0 or idx > sz, +NULL is returned. Additionally, array_ptr() of an out-of-bounds pointer is +not propagated to code which is speculatively executed. + +This can be used to protect the earlier load_array() example: + + int load_array(int *array, unsigned int idx) + { + int *elem; + + elem = array_ptr(array, idx, MAX_ARRAY_ELEMS); + if (elem) + return *elem; + else + return 0; + } + +This can also be used in situations where multiple fields on a structure are +accessed: + + struct foo array[SIZE]; + int a, b; + + void do_thing(int idx) + { + struct foo *elem; + + elem = array_ptr(array, idx, SIZE); + if (elem) { + a = elem->field_a; + b = elem->field_b; + } + } + +It is imperative that the returned pointer is used. Pointers which are +generated separately are subject to a number of potential CPU and compiler +optimizations, and may still be used speculatively. For example, this means +that the following sequence is unsafe: + + struct foo array[SIZE]; + int a, b; + + void do_thing(int idx) + { + if (array_ptr(array, idx, SIZE) != NULL) { + // unsafe as wrong pointer is used + a = array[idx].field_a; + b = array[idx].field_b; + } + } + +Similarly, it is unsafe to compare the returned pointer with other pointers, +as this may permit the compiler to substitute one pointer with another, +permitting speculation. For example, the following sequence is unsafe: + + struct foo array[SIZE]; + int a, b; + + void do_thing(int idx) + { + struct foo *elem = array_ptr(array, idx, size); + + // unsafe due to pointer substitution + if (elem == &array[idx]) { + a = elem->field_a; + b = elem->field_b; + } + } +