From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AG47ELvMLUP9IzEX3J/BCpJcPqFThx68Vbged/KKC2aCGfOVI7O+H/OdKRsxwXCksnNg8bqNjQy8 ARC-Seal: i=1; a=rsa-sha256; t=1519411252; cv=none; d=google.com; s=arc-20160816; b=DKCzEBLK3slUm1a1gAMYKsQLsMY4ZJ8JdSIuGp3WjCv/b4iBXMLvcFG1yUiSrwq7V2 TxCYaQj2i78nvs0a1+w7+3YYmC0mAe20wWucHLjw0GpL371q11zNqF5z97w9Yx1zAAGi aTLt0rE6aEy4Gfe4xjlzzuBF3MmjkRAl3kuO8A2KLyFUPPnXmTP1f4CzIeqQ4SKulWrQ 4Wky6rfa5PWRPdQ9/jxHQfPZrKVTkLX/Kmykwfs7g7EpkbRWWjSXyF7vE84u9+Sfxox1 fA9AHOIYd4bN+YiwEXoOeIsN8wq0K2Swtr+Gedz5+KuueOPUsaiHnTdNeMD63mzKR3PP mDkQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=mime-version:user-agent:references:in-reply-to:message-id:date :subject:cc:to:from:arc-authentication-results; bh=FbyFLFVtMqH4EXIpOD7ujpnnqAehzxdPOLiAEU/QEAg=; b=oJD9PC58p6kspZUss6KknYGGfKGtpN+bdqVsSkWNCsbEjZLRfAb88jonsF7eQ2Jr3c FkxtadGq4IAGgS1s//x+n5oSI1tXlMgCg+ibl1rzCMSvXCGOZvuRsNbL0Uljqxy3YBQH iHQMF0MT/LGcTdtXXe/Yf0vqxSNlla2Y3aajZ0LdoP5+WDNR+FOmrc478ysmAwT6z1uI 0y2ivFlqzhPYKspEsbAxrz3aYUTH8bQVbmL7xIEEXx84xwbwMdU2I8i/V90DL6U+wJ1M q1jJw7W28ulSoG7U2LD1E8Gz6YXQc7t5C2YDA2WjFurTbCotjwZahaycRAqIaqPmSnpO GP5g== ARC-Authentication-Results: i=1; mx.google.com; spf=softfail (google.com: domain of transitioning gregkh@linuxfoundation.org does not designate 90.92.71.90 as permitted sender) smtp.mailfrom=gregkh@linuxfoundation.org Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning gregkh@linuxfoundation.org does not designate 90.92.71.90 as permitted sender) smtp.mailfrom=gregkh@linuxfoundation.org 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 , Jack Wang Subject: [PATCH 4.4 173/193] Documentation: Document array_index_nospec Date: Fri, 23 Feb 2018 19:26:46 +0100 Message-Id: <20180223170353.126016056@linuxfoundation.org> X-Mailer: git-send-email 2.16.2 In-Reply-To: <20180223170325.997716448@linuxfoundation.org> References: <20180223170325.997716448@linuxfoundation.org> User-Agent: quilt/0.65 X-stable: review MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-LABELS: =?utf-8?b?IlxcU2VudCI=?= X-GMAIL-THRID: =?utf-8?q?1593218174234829623?= X-GMAIL-MSGID: =?utf-8?q?1593218174234829623?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 4.4-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 [jwang: cherry pick to 4.4] Signed-off-by: Jack Wang 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]; + } + }