From patchwork Fri Jan 5 14:57:48 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mark Rutland X-Patchwork-Id: 123528 Delivered-To: patch@linaro.org Received: by 10.140.22.227 with SMTP id 90csp910860qgn; Fri, 5 Jan 2018 06:58:13 -0800 (PST) X-Google-Smtp-Source: ACJfBouIspYejIMqwZD/QQ//rdaVW200ZttBU9GDAxm49IFFxMMdCd5skbT1eMKYmL8ukx5ofzgJ X-Received: by 10.101.82.138 with SMTP id y10mr2766065pgp.165.1515164293719; Fri, 05 Jan 2018 06:58:13 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1515164293; cv=none; d=google.com; s=arc-20160816; b=L0IdtJV6aqmBhYgfXALx9sRlEapNRN+zAqhroBfdxO1v+2aLpr2QWaLt9fVHR9TC6x xKAW9yKlQrl/hcx16OFAFPAEd9g4MBFlF7MhL7px1wCqPCeLiBPuWHJUWimH3xLEmFBw FD/Y+AEyvbTJd60/z37lq74ARnhk0KfdReJKcYMeHlcF6foX1dyCCMGkguuCVwdoybgY Y9FR9Hmr1WvxmF3+sr0Ux/FHi3TnRJYmROh47ZIxl1KhcvxFy4fvB7l3aZjnCCWLMgPI el9c5z01a9gnOSzrUWTrs8qBI/KgtVjM/UKkEjdG873yK+oe933cT38RT6hkgmeKb3q5 fyNw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:references:in-reply-to:message-id:date :subject:cc:to:from:arc-authentication-results; bh=4fy7ceuDtKNdGCY8zd3wm/eUWR452Osl0ALudt+piUU=; b=vzpFQYAjv/7Qzo5Pb+0Wf5mMX41hoRdtaDneXuCcXJG5hLl4vzSfjBiVC+tTCEX5MN 53SwQTpcT9exuw+jpmDYzMqhBseQXoDaG5Vpn/VQuSV/C9JmMFGIaZrZlhDLoFS8epJF KJH9qdbx+rdbbKpOw4+l4BNf2/HANqJZmPrWwDVhcDY6z3+d76ESL+D3Q2ks5LCm+mE6 sJudAN+oyQYU3pyD2MRYWmWhmuwTj/WapFHvuaMSNIlHDwY/FVkgIGJe1S5uVY8MliQm P13KPI0qDFrcyrMcEHA9ftDdZk7vW+qdill6xwR4gFBADQ9mRUwIThoXUiPv0JobhisE YJ3w== 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 33si4077516ply.308.2018.01.05.06.58.13; Fri, 05 Jan 2018 06:58:13 -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 S1752211AbeAEO6J (ORCPT + 26 others); Fri, 5 Jan 2018 09:58:09 -0500 Received: from foss.arm.com ([217.140.101.70]:46070 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752191AbeAEO6G (ORCPT ); Fri, 5 Jan 2018 09:58:06 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 36C8215BE; Fri, 5 Jan 2018 06:58:06 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 473E83F581; Fri, 5 Jan 2018 06:58:04 -0800 (PST) From: Mark Rutland To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Cc: dan.j.williams@intel.com, elena.reshetova@intel.com, corbet@lwn.net, alan@linux.intel.com, peterz@infradead.org, will.deacon@arm.com, gregkh@linuxfoundation.org, tglx@linutronix.de, Mark Rutland Subject: [RFCv2 2/4] Documentation: document nospec helpers Date: Fri, 5 Jan 2018 14:57:48 +0000 Message-Id: <20180105145750.53294-3-mark.rutland@arm.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20180105145750.53294-1-mark.rutland@arm.com> References: <20180105145750.53294-1-mark.rutland@arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Document the rationale and usage of the new nospec*() helpers. Signed-off-by: Mark Rutland Signed-off-by: Will Deacon Cc: Dan Williams Cc: Jonathan Corbet Cc: Peter Zijlstra --- Documentation/speculation.txt | 166 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 166 insertions(+) create mode 100644 Documentation/speculation.txt -- 2.11.0 diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..748fcd4dcda4 --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,166 @@ +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 following helpers found in can be used to prevent +information from being leaked via side-channels. + +* nospec_ptr(ptr, lo, hi) + + Returns a sanitized pointer that is bounded by the [lo, hi) interval. When + ptr < lo, or ptr >= hi, NULL is returned. Prevents an out-of-bounds pointer + being propagated to code which is speculatively executed. + + This is expected to be used by code which computes pointers to data + structures, where part of the address (such as an array index) may be + user-controlled. + + This can be used to protect the earlier load_array() example: + + int load_array(int *array, unsigned int idx) + { + int *elem; + + if ((elem = nospec_ptr(array + idx, array, array + MAX_ARRAY_ELEMS))) + 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; + + if ((elem = nospec_ptr(array + idx, array, array + SIZE)) { + 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 (nospec_ptr(array + idx, array, array + 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 = nospec_ptr(array + idx, array, array + size); + + // unsafe due to pointer substitution + if (elem == &array[idx]) { + a = elem->field_a; + b = elem->field_b; + } + } + +* nospec_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. Prevents an + out-of-bounds pointer being propagated to code which is speculatively + executed. + + This is a convenience function which wraps nospec_ptr(), and has the same + caveats w.r.t. the use of the returned pointer. + + For example, this may be used as follows: + + int load_array(int *array, unsigned int idx) + { + int *elem; + + if ((elem = nospec_array_ptr(array, idx, MAX_ARRAY_ELEMS))) + return *elem; + else + return 0; + } +