From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation Date: Tue, 06 Feb 2018 09:14:48 +0000 Message-ID: <1517908488.31953.198.camel@infradead.org> References: <1516476182-5153-10-git-send-email-karahmed@amazon.de> <1516566497.9814.78.camel@infradead.org> <1516572013.9814.109.camel@infradead.org> <1516638426.9521.20.camel@infradead.org> <20180123072930.soz25cyky3u4hpgv@gmail.com> <20180123075358.nztpyxympwfkyi2a@gmail.com> <1516699832.9521.123.camel@infradead.org> <20180123102318.airsvcl5uckguo2z@gmail.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-6fBrD9xtQM+ZSF9EkIi1" Cc: Linus Torvalds , KarimAllah Ahmed , Linux Kernel Mailing List , Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Arjan van de Ven , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , To: Thomas Gleixner , Ingo Molnar Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org --=-6fBrD9xtQM+ZSF9EkIi1 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sun, 2018-02-04 at 19:43 +0100, Thomas Gleixner wrote: > Yet another possibility is to avoid the function entry and accouting magi= c > and use the generic gcc return thunk: >=20 > __x86_return_thunk: > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0call L2 > L1: > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0pause > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0lfence > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0jmp L1 > L2: > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0lea 8(%rsp), %rsp|lea 4(%= esp), %esp > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ret >=20 > which basically refills the RSB on every return. That can be inline or > extern, but in both cases we should be able to patch it out. >=20 > I have no idea how that affects performance, but it might be worthwhile t= o > experiment with that. That was what I had in mind when I asked HJ to add -mfunction-return. I suspect the performance hit would be significant because it would cause a prediction miss on *every* return. But as I said, let's implement what we can without IBRS for Skylake, then we can compare the two options for performance, security coverage and general fugliness. --=-6fBrD9xtQM+ZSF9EkIi1 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 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