From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Cyrus-Session-Id: sloti22d1t05-1465196-1518185194-2-7512863963730737900 X-Sieve: CMU Sieve 3.0 X-Spam-known-sender: no X-Spam-score: 0.0 X-Spam-hits: BAYES_00 -1.9, HEADER_FROM_DIFFERENT_DOMAINS 0.001, ME_NOAUTH 0.01, RCVD_IN_DNSWL_HI -5, T_RP_MATCHES_RCVD -0.01, LANGUAGES en, BAYES_USED global, SA_VERSION 3.4.0 X-Spam-source: IP='209.132.180.67', Host='vger.kernel.org', Country='US', FromHeader='org', MailFrom='org' X-Spam-charsets: plain='UTF-8' X-Resolved-to: greg@kroah.com X-Delivered-to: greg@kroah.com X-Mail-from: stable-owner@vger.kernel.org ARC-Seal: i=1; a=rsa-sha256; cv=none; d=messagingengine.com; s=arctest; t=1518185192; b=YLF/Gzhi7F2SwrOPIUWCG7GvPbX2cBlJ2VL00aGE2DRTusk /tWRYddGkwtq1RuvzIeEHjWgt2PjHTQZ8LNfKsPdKaeQ/OP9x3uLyD6aQLSGI0zD ztRlulytOQoHcens0mn9ZBmkdtiKuYcKJpLqsCjg8GNu5fh+0Z4t/v+G9rNgkDbk 0FQDLpUiuuADNsHj3AZ0+iHiCEspNWMjyPIC/tLFt682bolU2PgiPpiadmsXZBD4 HdAjRekOxsu/6fKlKyFM4l3ydhwgLk3HZ45su3zP9pqIFdjzXGoaLQ6XhkHBfQTd Zbar2XShErJPiMg2FGYD7B+V4j94zC3idJvGpyw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=from:to:cc:subject:date:message-id :in-reply-to:references:mime-version:content-type:sender :list-id; s=arctest; t=1518185192; bh=68sidBXRhfeKSC1Y7RWV2FTYwW mdfPLcsajhuCVHN2s=; b=XtYyC8XwOER7VhQ9hv2QGc5F33mOLxAr4JIsH0ZWYT 3F4GT1Xp30kL+nlc2YlxRoE4ld6wPSPbpDnVK9NfOid3PuSjlMwzwubzO7lmsRdc 5pO96wpOqnnYrHBVElSUGhSk5lTfmhrSMMKzUrMUBQ3fbYGu8aXJfun0HvRbXUqR eP323AIL56RFOfCwfH9syv/16ysYJpZNbc3cyaI1paJLBMoS3mZEucqkezguaTv2 ezr929MSCS1P4BynJqT/aS8tcorLe61PczVD+yccQcJFnhi4uSG8ZPzjH91QzdRK D5JVHDNANehkbG8+sX7LR6a8Hm/IrL2eZ5L82LPVYKlg== ARC-Authentication-Results: i=1; mx2.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=none (p=none,has-list-id=yes,d=none) header.from=linuxfoundation.org; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=linuxfoundation.org header.result=pass header_is_org_domain=yes Authentication-Results: mx2.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=none (p=none,has-list-id=yes,d=none) header.from=linuxfoundation.org; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=linuxfoundation.org header.result=pass header_is_org_domain=yes Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753067AbeBINmu (ORCPT ); Fri, 9 Feb 2018 08:42:50 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:50000 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753028AbeBINms (ORCPT ); Fri, 9 Feb 2018 08:42:48 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Mark Rutland , Will Deacon , Dan Williams , Thomas Gleixner , Kees Cook , linux-arch@vger.kernel.org, Jonathan Corbet , Peter Zijlstra , kernel-hardening@lists.openwall.com, torvalds@linux-foundation.org, alan@linux.intel.com, David Woodhouse Subject: [PATCH 4.9 56/92] Documentation: Document array_index_nospec Date: Fri, 9 Feb 2018 14:39:25 +0100 Message-Id: <20180209133935.257746808@linuxfoundation.org> X-Mailer: git-send-email 2.16.1 In-Reply-To: <20180209133931.211869118@linuxfoundation.org> References: <20180209133931.211869118@linuxfoundation.org> User-Agent: quilt/0.65 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Mark Rutland (cherry picked from commit f84a56f73dddaeac1dba8045b007f742f61cd2da) Document the rationale and usage of the new array_index_nospec() helper. Signed-off-by: Mark Rutland Signed-off-by: Will Deacon Signed-off-by: Dan Williams Signed-off-by: Thomas Gleixner Reviewed-by: Kees Cook Cc: linux-arch@vger.kernel.org Cc: Jonathan Corbet Cc: Peter Zijlstra Cc: gregkh@linuxfoundation.org Cc: kernel-hardening@lists.openwall.com Cc: torvalds@linux-foundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727413645.33451.15878817161436755393.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: David Woodhouse Signed-off-by: Greg Kroah-Hartman --- Documentation/speculation.txt | 90 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 90 insertions(+) create mode 100644 Documentation/speculation.txt --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,90 @@ +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 index) + { + if (index >= MAX_ARRAY_ELEMS) + return 0; + else + return array[index]; + } + +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[index], even if index >= 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 index) + { + int val1, val2, + + val1 = load_array(arr1, index); + 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_index_nospec() helper in can be used to +prevent information from being leaked via side-channels. + +A call to array_index_nospec(index, size) returns a sanitized index +value that is bounded to [0, size) even under cpu speculation +conditions. + +This can be used to protect the earlier load_array() example: + + int load_array(int *array, unsigned int index) + { + if (index >= MAX_ARRAY_ELEMS) + return 0; + else { + index = array_index_nospec(index, MAX_ARRAY_ELEMS); + return array[index]; + } + }