From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: ACJfBouhQGVEUujVetp+wcYOaHkNsx+vD3H9evHkZzU+LtkogPXnyMINeu1Ob2rtq2MmMJvqlQus ARC-Seal: i=1; a=rsa-sha256; t=1515460734; cv=none; d=google.com; s=arc-20160816; b=SRgKNofZ2fgJS/XcDpEJPLqfVi+WVFpHLb4eGXWF4REqx1mmaFSgHGvO7yGtm9DSVd rq52JVRcrW1h69dgw9cPHEe7ZgJ2EDhoUhYcybGq3wd4uKtwXv+bY2RrydTobMkYYCF2 i8AIOxVEiaQ12vYupAQ1z2dQGk+CQbfz8E6hV37fjoD5/uEgMPeVA1r6GN4s2npBIyTW jC5s1Jcy+JlSBiqc+oIbpsHnv9PMdv2Iv5yCEbBIz0NaBo6MNb/XU2DYYGjUf5FYCTCB hohCqYEmXH30TtItXTLzJrJXu5IEwnzU0t9ypaHH7noOUPS1TZV6CIpSSoQHkEJsZL9S Lsog== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=mime-version:accept-language:in-reply-to:references:message-id:date :thread-index:thread-topic:subject:cc:to:from:mime-version :dkim-signature:arc-authentication-results; bh=fHN/AeNCQI8d8kniTJe3roBMkIT9YQ998Pza0q+plvI=; b=XQ6gwQrdBdnHMfcMTUTS3BXRiwYfB1cdBl2adIwq+771pFHe/193q5twckSYzGsI/X kNbTJzKU8qJU3rK/I97RQhAotUtv4W6Gwh3GX1NB+Lxcai7uBLqSfq74Jvh8pmXuHczm sP11O/VuEGRnruQadB4DkQF2ytiYGk9bvnEXRTHtnIjlEPk2Yd6oG+VkAOCvCeKYJ3qe b8IvsqURZOuAHZ1rIerx74iW7gQCMj7q3PpAr7Tdx/sniMIIgmzhoMHLV/vV+o3v9eNB ukMVQxMC413xe8CvqNLnsFv6R+jagKlcWqjCtaFtZtMzZTKSVoT9IjPLQ8tM/gejnfJB QYrA== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@amazon.co.uk header.s=amazon201209 header.b=L1Z+ja1Q; spf=pass (google.com: domain of prvs=5408f6f94=dwmw@amazon.co.uk designates 52.95.48.154 as permitted sender) smtp.mailfrom=prvs=5408f6f94=dwmw@amazon.co.uk; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=amazon.co.uk Authentication-Results: mx.google.com; dkim=pass header.i=@amazon.co.uk header.s=amazon201209 header.b=L1Z+ja1Q; spf=pass (google.com: domain of prvs=5408f6f94=dwmw@amazon.co.uk designates 52.95.48.154 as permitted sender) smtp.mailfrom=prvs=5408f6f94=dwmw@amazon.co.uk; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=amazon.co.uk X-Amazon-filename: smime.p7s X-IronPort-AV: E=Sophos;i="5.46,333,1511827200"; d="p7s'?scan'208";a="326931818" Content-Type: multipart/mixed; boundary="===============3484936004075506750==" MIME-Version: 1.0 From: "Woodhouse, David" To: "torvalds@linux-foundation.org" , "ak@linux.intel.com" CC: "linux-kernel@vger.kernel.org" , "peterz@infradead.org" , "tim.c.chen@linux.intel.com" , "tglx@linutronix.de" , "riel@redhat.com" , "keescook@google.com" , "gnomes@lxorguk.ukuu.org.uk" , "pjt@google.com" , "dave.hansen@intel.com" , "luto@amacapital.net" , "jikos@kernel.org" , "gregkh@linux-foundation.org" Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch Thread-Topic: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch Thread-Index: AQHTiNqwa2C9WetvxEOBENb6z+MLpqNqpuUAgAAAhgCAAAphgIAAAnGAgAAD7oCAAAWigA== Date: Tue, 9 Jan 2018 01:18:30 +0000 Message-ID: <1515460709.4423.101.camel@amazon.co.uk> References: <1515363085-4219-1-git-send-email-dwmw@amazon.co.uk> <1515455051.15588.7.camel@infradead.org> <1515455902.4423.59.camel@amazon.co.uk> <20180109004415.GG6718@tassilo.jf.intel.com> In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: yes X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.43.62.4] MIME-Version: 1.0 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1589069808671539027?= X-GMAIL-MSGID: =?utf-8?q?1589075755138240615?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: --===============3484936004075506750== Content-Language: en-US Content-Type: multipart/signed; micalg=sha-256; protocol="application/x-pkcs7-signature"; boundary="=-3k4SJf4YBj+/W9o1sPle" --=-3k4SJf4YBj+/W9o1sPle Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-08 at 16:58 -0800, Linus Torvalds wrote: >=20 > And THAT is where "clear the registers" comes in. It adds _another_ > huge barrier to an attack that was already pretty hard to begin with. >=20 > If we clear the registers, what the hell are you going to put in the > RSB that helps you? As you go back up the call stack, a lot of caller-saved registers are going to end up being reloaded, before you hit the mispredicted 'ret'. Much of those could be attacker-controlled, by data passed in to the syscall or pre-populated in other data structures. Clearing the call-clobbered registers in context switch will probably eliminate a few attack vectors, but I'm not sure it's even the majority. --=-3k4SJf4YBj+/W9o1sPle Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEE4w ggUuMIIEFqADAgECAhEApEtL31g6H7ROXdOiQnA9RjANBgkqhkiG9w0BAQsFADCBlzELMAkGA1UE BhMCR0IxGzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgG A1UEChMRQ09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhl bnRpY2F0aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0EwHhcNMTcxMjIxMDAwMDAwWhcNMTgxMjIxMjM1 OTU5WjAiMSAwHgYJKoZIhvcNAQkBFhFkd213QGFtYXpvbi5jby51azCCASIwDQYJKoZIhvcNAQEB BQADggEPADCCAQoCggEBAKdGKgXuwKMg2r+i/4BZZC0ddRxNq3xIKTakie/VCSzoO7P17A36ZzUc VMEYPfqDt/65xoc6Tdih+qkY2pNDppZ1DZ8mVrAX6O2O60ZhmXB60wMoDvXPZInvkMOW4drqnje/ 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