Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755246AbeAIDFq (ORCPT + 1 other); Mon, 8 Jan 2018 22:05:46 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:42100 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755157AbeAIDFp (ORCPT ); Mon, 8 Jan 2018 22:05:45 -0500 Message-ID: <1515467124.22302.8.camel@infradead.org> Subject: Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch From: David Woodhouse To: Paul Turner Cc: Peter Zijlstra , "Van De Ven, Arjan" , Andi Kleen , LKML , Greg Kroah-Hartman , Tim Chen , Dave Hansen , Thomas Gleixner , Andy Lutomirski , Andi Kleen In-Reply-To: References: <20180108201531.6782-1-andi@firstfloor.org> <20180108221121.GU29822@worktop.programming.kicks-ass.net> <1515458673.4423.79.camel@amazon.co.uk> <1515458882.4423.82.camel@infradead.org> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-/Ta5dCEOXm4tU0Qu4H/6" Date: Tue, 09 Jan 2018 03:05:24 +0000 Mime-Version: 1.0 X-Mailer: Evolution 3.18.5.2-0ubuntu3.2 X-SRS-Rewrite: SMTP reverse-path rewritten from by twosheds.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: --=-/Ta5dCEOXm4tU0Qu4H/6 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-08 at 18:48 -0800, Paul Turner wrote: > On Mon, Jan 8, 2018 at 4:48 PM, David Woodhouse wro= te: > >=20 > > On Tue, 2018-01-09 at 00:44 +0000, Woodhouse, David wrote: > > >=20 > > > On IRC, Arjan assures me that 'pause' here really is sufficient as a > > > speculation trap. If we do end up returning back here as a > > > misprediction, that 'pause' will stop the speculative execution on > > > affected CPUs even though it isn't *architecturally* documented to do > > > so. > > >=20 > > > Arjan, can you confirm that in email please? > >=20 > > That actually doesn't make sense to me. If 'pause' alone is sufficient, > > then why in $DEITY's name would we need a '1:pause;jmp 1b' loop in the > > retpoline itself? > >=20 > > Arjan? > On further investigation, I don't understand any of the motivation for > the changes here: > - It micro-benchmarks several cycles slower than the suggested > implementation on average (38 vs 44 cycles) [likely due to lost 16-byte c= all > alignment] > - It's much larger in terms of .text size (120 bytes @ 16 calls, 218 > bytes @ 30 calls) vs (61 bytes) > - I'm not sure it's universally correct in preventing speculation: >=20 > (1) I am able to observe a small timing difference between executing > "1: pause; jmp 1b;" and "pause" in the speculative path. > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Given that alignment is otherwise identical= , this should only > occur if execution is non-identical, which would require speculative > execution to proceed beyond the pause. > (2) When we proposed and reviewed the sequence.=C2=A0=C2=A0This was not c= ited by > architects as a way of presenting speculation.=C2=A0=C2=A0Indeed, as Davi= d > points out, we'd consider using this within the sequence without the > loop. >=20 >=20 > If the claim above is true -- which (1) actually appears to contradict > -- it seems to bear stronger validation.=C2=A0=C2=A0Particularly since th= at in > the suggested sequences we can fit the jmps within the space we get > for free by aligning the call targets. Some of the discrepancies are because it's been filtered through Intel and I may not have had your latest version. I'm going to revert to your version from https://support.google.com/faqs/answer/7625886=C2=A0=E2=80=94 for the retpo= line (i.e. adding back the alignment) and the RSB stuffing. If Intel have a sequence which is simpler and guaranteed to work, I'll let them post that with an authoritative statement from the CPU architects. At this point, we really need to get on with rolling in the other parts on top. --=-/Ta5dCEOXm4tU0Qu4H/6 Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQDgzLNWa18DNpGUj/ZeH0Sgz53ESIbzdPw3OJeuNP6jZhxZojbyfxbM hETscxI/Hj6UZ4a7sHm5BkVjlsB1Af2Za/PXUt8MmLAcPMHkMPGunvkUibEvblDvpqMkQZlaZM+t 5PqFmWkbehLaEvbpNY7dmEAAeKh4klTzJzrr5AAzaCQ32cA2e3+DEIv5O5l9ViMIjy/JM+xMQrfX 3PZ0chY1PaVWjg59d4Uno+5LRDbgCnPkKJX4ysBGadibjBGQGJEZCjh94iiEebn2KsRLvtrJ72Ph 3W2HDEdngW3YP0wujFQVs81U7L8XN3kdPRsa9zNqGtYQP/+1KMMJQ57hnfi9AgMBAAGjggHpMIIB 5TAfBgNVHSMEGDAWgBSCr2yM+MX+lmF86B89K3FIXsSLwDAdBgNVHQ4EFgQUpL+/5lli9jmj2KHj ryyhnB2xRt0wDgYDVR0PAQH/BAQDAgWgMAwGA1UdEwEB/wQCMAAwIAYDVR0lBBkwFwYIKwYBBQUH AwQGCysGAQQBsjEBAwUCMBEGCWCGSAGG+EIBAQQEAwIFIDBGBgNVHSAEPzA9MDsGDCsGAQQBsjEB AgEBATArMCkGCCsGAQUFBwIBFh1odHRwczovL3NlY3VyZS5jb21vZG8ubmV0L0NQUzBaBgNVHR8E UzBRME+gTaBLhklodHRwOi8vY3JsLmNvbW9kb2NhLmNvbS9DT01PRE9SU0FDbGllbnRBdXRoZW50 aWNhdGlvbmFuZFNlY3VyZUVtYWlsQ0EuY3JsMIGLBggrBgEFBQcBAQR/MH0wVQYIKwYBBQUHMAKG SWh0dHA6Ly9jcnQuY29tb2RvY2EuY29tL0NPTU9ET1JTQUNsaWVudEF1dGhlbnRpY2F0aW9uYW5k U2VjdXJlRW1haWxDQS5jcnQwJAYIKwYBBQUHMAGGGGh0dHA6Ly9vY3NwLmNvbW9kb2NhLmNvbTAe BgNVHREEFzAVgRNkd213MkBpbmZyYWRlYWQub3JnMA0GCSqGSIb3DQEBCwUAA4IBAQCK28BdbVJ9 QKQqTDfXwogAYiRBEGptfE1Bjy4F5vC6eWJqOJ15vunxjLwdbZYb4L0qrJlh+ZHHHlbIK8uEZu7N XHUntmWMbGbZiu7JgrbSXJK1ct9gxrN/sdWYJ+JDjVHg7GfDTvTTPa26JMRqJsO1TjjyDX7A3K39 TjV8C0hqXvwF9BsNf+qBeWO6GVzJ5572awY221hc1umibmZaKV4fg+7fS7qscx5TSuIc6uvMBQhm 7NQiCq6euMMWBDUDlotQCDW0ilm0OuLW3IVLuZCm6Msc+6hT9+dCT4JUvxTHZnnO7uLCxV+Ujad+ PH3itRm38i96p2zvwgLr8vwWA0ckMIIFMTCCBBmgAwIBAgIQTUYRMpPx2e52jnkx1q87gDANBgkq hkiG9w0BAQsFADCBlzELMAkGA1UEBhMCR0IxGzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQ MA4GA1UEBxMHU2FsZm9yZDEaMBgGA1UEChMRQ09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENP TU9ETyBSU0EgQ2xpZW50IEF1dGhlbnRpY2F0aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0EwHhcNMTcx MjIxMDAwMDAwWhcNMTgxMjIxMjM1OTU5WjAkMSIwIAYJKoZIhvcNAQkBFhNkd213MkBpbmZyYWRl YWQub3JnMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA4MyzVmtfAzaRlI/2Xh9EoM+d xEiG83T8NziXrjT+o2YcWaI28n8WzIRE7HMSPx4+lGeGu7B5uQZFY5bAdQH9mWvz11LfDJiwHDzB 5DDxrp75FImxL25Q76ajJEGZWmTPreT6hZlpG3oS2hL26TWO3ZhAAHioeJJU8yc66+QAM2gkN9nA Nnt/gxCL+TuZfVYjCI8vyTPsTEK319z2dHIWNT2lVo4OfXeFJ6PuS0Q24Apz5CiV+MrARmnYm4wR kBiRGQo4feIohHm59irES77aye9j4d1thwxHZ4Ft2D9MLoxUFbPNVOy/Fzd5HT0bGvczahrWED// tSjDCUOe4Z34vQIDAQABo4IB6TCCAeUwHwYDVR0jBBgwFoAUgq9sjPjF/pZhfOgfPStxSF7Ei8Aw HQYDVR0OBBYEFKS/v+ZZYvY5o9ih468soZwdsUbdMA4GA1UdDwEB/wQEAwIFoDAMBgNVHRMBAf8E AjAAMCAGA1UdJQQZMBcGCCsGAQUFBwMEBgsrBgEEAbIxAQMFAjARBglghkgBhvhCAQEEBAMCBSAw RgYDVR0gBD8wPTA7BgwrBgEEAbIxAQIBAQEwKzApBggrBgEFBQcCARYdaHR0cHM6Ly9zZWN1cmUu Y29tb2RvLm5ldC9DUFMwWgYDVR0fBFMwUTBPoE2gS4ZJaHR0cDovL2NybC5jb21vZG9jYS5jb20v Q09NT0RPUlNBQ2xpZW50QXV0aGVudGljYXRpb25hbmRTZWN1cmVFbWFpbENBLmNybDCBiwYIKwYB BQUHAQEEfzB9MFUGCCsGAQUFBzAChklodHRwOi8vY3J0LmNvbW9kb2NhLmNvbS9DT01PRE9SU0FD bGllbnRBdXRoZW50aWNhdGlvbmFuZFNlY3VyZUVtYWlsQ0EuY3J0MCQGCCsGAQUFBzABhhhodHRw Oi8vb2NzcC5jb21vZG9jYS5jb20wHgYDVR0RBBcwFYETZHdtdzJAaW5mcmFkZWFkLm9yZzANBgkq hkiG9w0BAQsFAAOCAQEAitvAXW1SfUCkKkw318KIAGIkQRBqbXxNQY8uBebwunliajideb7p8Yy8 HW2WG+C9KqyZYfmRxx5WyCvLhGbuzVx1J7ZljGxm2YruyYK20lyStXLfYMazf7HVmCfiQ41R4Oxn w0700z2tuiTEaibDtU448g1+wNyt/U41fAtIal78BfQbDX/qgXljuhlcyeee9msGNttYXNbpom5m WileH4Pu30u6rHMeU0riHOrrzAUIZuzUIgqunrjDFgQ1A5aLUAg1tIpZtDri1tyFS7mQpujLHPuo U/fnQk+CVL8Ux2Z5zu7iwsVflI2nfjx94rUZt/Iveqds78IC6/L8FgNHJDCCBeYwggPOoAMCAQIC EGqb4Tg7/ytrnwHV2binUlYwDQYJKoZIhvcNAQEMBQAwgYUxCzAJBgNVBAYTAkdCMRswGQYDVQQI ExJHcmVhdGVyIE1hbmNoZXN0ZXIxEDAOBgNVBAcTB1NhbGZvcmQxGjAYBgNVBAoTEUNPTU9ETyBD QSBMaW1pdGVkMSswKQYDVQQDEyJDT01PRE8gUlNBIENlcnRpZmljYXRpb24gQXV0aG9yaXR5MB4X DTEzMDExMDAwMDAwMFoXDTI4MDEwOTIzNTk1OVowgZcxCzAJBgNVBAYTAkdCMRswGQYDVQQIExJH cmVhdGVyIE1hbmNoZXN0ZXIxEDAOBgNVBAcTB1NhbGZvcmQxGjAYBgNVBAoTEUNPTU9ETyBDQSBM aW1pdGVkMT0wOwYDVQQDEzRDT01PRE8gUlNBIENsaWVudCBBdXRoZW50aWNhdGlvbiBhbmQgU2Vj dXJlIEVtYWlsIENBMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvrOeV6wodnVAFsc4 A5jTxhh2IVDzJXkLTLWg0X06WD6cpzEup/Y0dtmEatrQPTRI5Or1u6zf+bGBSyD9aH95dDSmeny1 nxdlYCeXIoymMv6pQHJGNcIDpFDIMypVpVSRsivlJTRENf+RKwrB6vcfWlP8dSsE3Rfywq09N0Zf xcBa39V0wsGtkGWC+eQKiz4pBZYKjrc5NOpG9qrxpZxyb4o4yNNwTqzaaPpGRqXB7IMjtf7tTmU2 jqPMLxFNe1VXj9XB1rHvbRikw8lBoNoSWY66nJN/VCJv5ym6Q0mdCbDKCMPybTjoNCQuelc0IAaO 4nLUXk0BOSxSxt8kCvsUtQIDAQABo4IBPDCCATgwHwYDVR0jBBgwFoAUu69+Aj36pvE8hI6t7jiY 7NkyMtQwHQYDVR0OBBYEFIKvbIz4xf6WYXzoHz0rcUhexIvAMA4GA1UdDwEB/wQEAwIBhjASBgNV HRMBAf8ECDAGAQH/AgEAMBEGA1UdIAQKMAgwBgYEVR0gADBMBgNVHR8ERTBDMEGgP6A9hjtodHRw Oi8vY3JsLmNvbW9kb2NhLmNvbS9DT01PRE9SU0FDZXJ0aWZpY2F0aW9uQXV0aG9yaXR5LmNybDBx BggrBgEFBQcBAQRlMGMwOwYIKwYBBQUHMAKGL2h0dHA6Ly9jcnQuY29tb2RvY2EuY29tL0NPTU9E T1JTQUFkZFRydXN0Q0EuY3J0MCQGCCsGAQUFBzABhhhodHRwOi8vb2NzcC5jb21vZG9jYS5jb20w DQYJKoZIhvcNAQEMBQADggIBAHhcsoEoNE887l9Wzp+XVuyPomsX9vP2SQgG1NgvNc3fQP7TcePo 7EIMERoh42awGGsma65u/ITse2hKZHzT0CBxhuhb6txM1n/y78e/4ZOs0j8CGpfb+SJA3GaBQ+39 4k+z3ZByWPQedXLL1OdK8aRINTsjk/H5Ns77zwbjOKkDamxlpZ4TKSDMKVmU/PUWNMKSTvtlenlx Bhh7ETrN543j/Q6qqgCWgWuMAXijnRglp9fyadqGOncjZjaaSOGTTFB+E2pvOUtY+hPebuPtTbq7 vODqzCM6ryEhNhzf+enm0zlpXK7q332nXttNtjv7VFNYG+I31gnMrwfHM5tdhYF/8v5UY5g2xANP ECTQdu9vWPoqNSGDt87b3gXb1AiGGaI06vzgkejL580ul+9hz9D0S0U4jkhJiA7EuTecP/CFtR72 uYRBcunwwH3fciPjviDDAI9SnC/2aPY8ydehzuZutLbZdRJ5PDEJM/1tyZR2niOYihZ+FCbtf3D9 mB12D4ln9icgc7CwaxpNSCPt8i/GqK2HsOgkL3VYnwtx7cJUmpvVdZ4ognzgXtgtdk3ShrtOS1iA N2ZBXFiRmjVzmehoMof06r1xub+85hFQzVxZx5/bRaTKTlL8YXLI8nAbR9HWdFqzcOoB/hxfEyIQ px9/s81rgzdEZOofSlZHynoSMYIDxzCCA8MCAQEwgawwgZcxCzAJBgNVBAYTAkdCMRswGQYDVQQI ExJHcmVhdGVyIE1hbmNoZXN0ZXIxEDAOBgNVBAcTB1NhbGZvcmQxGjAYBgNVBAoTEUNPTU9ETyBD QSBMaW1pdGVkMT0wOwYDVQQDEzRDT01PRE8gUlNBIENsaWVudCBBdXRoZW50aWNhdGlvbiBhbmQg U2VjdXJlIEVtYWlsIENBAhBNRhEyk/HZ7naOeTHWrzuAMA0GCWCGSAFlAwQCAQUAoIIB6zAYBgkq hkiG9w0BCQMxCwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0xODAxMDkwMzA1MjRaMC8GCSqG SIb3DQEJBDEiBCBGtwG5iHALz5kyST6rZENBx9WwMLv/lxuTcqsLiLRoLTCBvQYJKwYBBAGCNxAE MYGvMIGsMIGXMQswCQYDVQQGEwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYD VQQHEwdTYWxmb3JkMRowGAYDVQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RP IFJTQSBDbGllbnQgQXV0aGVudGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQQIQTUYRMpPx2e52 jnkx1q87gDCBvwYLKoZIhvcNAQkQAgsxga+ggawwgZcxCzAJBgNVBAYTAkdCMRswGQYDVQQIExJH cmVhdGVyIE1hbmNoZXN0ZXIxEDAOBgNVBAcTB1NhbGZvcmQxGjAYBgNVBAoTEUNPTU9ETyBDQSBM aW1pdGVkMT0wOwYDVQQDEzRDT01PRE8gUlNBIENsaWVudCBBdXRoZW50aWNhdGlvbiBhbmQgU2Vj dXJlIEVtYWlsIENBAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBAQUABIIBAB5drG8+hR93 JvnSfos6GUZ/skpKJORo9TAdKrJGx1pWGayb4KlsiNOPtDnhDOGXCtGcCeVhki94KZja+ihpcaD1 jS9bv0zt8nXgu1D1YzquoMzRnM/QIbAzr89cRucvzsev6PjXd9+ywu9UXEfF11nUJaFcFsGp3M0E N8czBlY8zPmXr4h/Bh1I+UtdvDLTnqUxnsc59hkt4lM8hI9ljna6r+FBwILGo5/0mkxHOLvTtmsW vdB5/HAKAvm2EA5Jp/NZIWpd0OhKvBupS1xDPxvVtPqlhRERMih9J20ysu1n8xhklN+bYBeCwLxz qv7TSxqtuk0zlacvIhZZalLAQV8AAAAAAAA= --=-/Ta5dCEOXm4tU0Qu4H/6--