From patchwork Fri Jan 12 00:46:30 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Dan Williams X-Patchwork-Id: 124242 Delivered-To: patch@linaro.org Received: by 10.140.22.227 with SMTP id 90csp1432778qgn; Thu, 11 Jan 2018 16:54:49 -0800 (PST) X-Google-Smtp-Source: ACJfBovZDeVv3GoCTiJ7lgh4D+GU/GhECXy5K3SxeaI3VAbFYepnbZ7qdA8LCxaQSe6YM/CQzbDh X-Received: by 10.98.247.19 with SMTP id h19mr1451685pfi.77.1515718489865; Thu, 11 Jan 2018 16:54:49 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1515718489; cv=none; d=google.com; s=arc-20160816; b=AYmyX54V6rCg20Jvp9SK1/oqw1KOGSx13na7fNXYo53ktroSu0+a3YjersaBmvo24l lZq9qIMf+zm4aU06EymENjZxUeUIeCjuWaPpAwUYCiS/bSRItvV94NNTsVCfpGxbrLWM aRoj5ITT9qKjOUrdERZUcU6JY954WRah+u3UGRQG1f11nizbmzmo3a6WygQwNJo0PfZm E6X1znk7XEmCa5RGLi+p0AeejUyBZfIGlj3Tjs+HwKMCqET4rTGxqAw/zle70QLXb3+f pIsT5dhW8Z0l2yqFU9gh4JSO863bluW/gYxvpoSIHUwTOIpvmOw2qTKQy7ULuVZr1JwO JQVQ== 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=EsM9gikBqpCAHJ9y6fAKmrOx2BpVdiLWEi7F4mlucXo=; b=i/NqbsnlWZ0Got52HY84MYnVdKuFUFfnVVSPQoC6TDugjAdMKA1PQDcGhUVXvkAhRo 5liR7BWBTeQESs0EYU29NbWu161xKJp/PylW6zSSQey45widSYyFcbZg3bqd9I0Pn0BP Eexg11rAOA0QASfSkCiQkIjsRXBOfdThkTn6vobYcUbBLA0IE0TswHP1phdqDzDul3os WMb6ftx7lo3fFnqpAgDEfxNVpnBJgJzG6W5YS/xKvB/YAU5PtDK/IcE2N4UMra7oly0a qPB7USb8950RnNAnCOqYSulW1hj+kAeqoFSl6f2VQ88FpGDhFwRbyzgURxNqC4m/8iFX pvUw== 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 e5si12753727pgv.415.2018.01.11.16.54.49; Thu, 11 Jan 2018 16:54:49 -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 S932586AbeALAys (ORCPT + 28 others); Thu, 11 Jan 2018 19:54:48 -0500 Received: from mga07.intel.com ([134.134.136.100]:16015 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932148AbeALAyp (ORCPT ); Thu, 11 Jan 2018 19:54:45 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Jan 2018 16:54:44 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,346,1511856000"; d="scan'208";a="9502958" Received: from dwillia2-desk3.jf.intel.com (HELO dwillia2-desk3.amr.corp.intel.com) ([10.54.39.16]) by fmsmga002.fm.intel.com with ESMTP; 11 Jan 2018 16:54:43 -0800 Subject: [PATCH v2 01/19] Documentation: document array_ptr From: Dan Williams To: linux-kernel@vger.kernel.org Cc: Mark Rutland , linux-arch@vger.kernel.org, kernel-hardening@lists.openwall.com, Peter Zijlstra , Jonathan Corbet , Will Deacon , tglx@linutronix.de, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@linux.intel.com Date: Thu, 11 Jan 2018 16:46:30 -0800 Message-ID: <151571799008.27429.12325141216769795517.stgit@dwillia2-desk3.amr.corp.intel.com> In-Reply-To: <151571798296.27429.7166552848688034184.stgit@dwillia2-desk3.amr.corp.intel.com> References: <151571798296.27429.7166552848688034184.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 Signed-off-by: Dan Williams --- Documentation/speculation.txt | 142 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 142 insertions(+) create mode 100644 Documentation/speculation.txt Reviewed-by: Kees Cook diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..a4d465fd42cd --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,142 @@ +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() an out-of-bounds poitner 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; + } + } +