Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758716AbeAIAmg (ORCPT + 1 other); Mon, 8 Jan 2018 19:42:36 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:32940 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750913AbeAIAmc (ORCPT ); Mon, 8 Jan 2018 19:42:32 -0500 Message-ID: <1515458545.4423.76.camel@infradead.org> Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch From: David Woodhouse To: Linus Torvalds Cc: "linux-kernel@vger.kernel.org" , "tim.c.chen@linux.intel.com" , "peterz@infradead.org" , "tglx@linutronix.de" , "ak@linux.intel.com" , "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" In-Reply-To: References: <1515363085-4219-1-git-send-email-dwmw@amazon.co.uk> <1515455051.15588.7.camel@infradead.org> <1515455902.4423.59.camel@amazon.co.uk> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-WMUDCYvaMVoWGQSaqWxk" Date: Tue, 09 Jan 2018 00:42:25 +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: --=-WMUDCYvaMVoWGQSaqWxk Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-08 at 16:35 -0800, Linus Torvalds wrote: > On Mon, Jan 8, 2018 at 3:58 PM, Woodhouse, David wrot= e: > >> > >> Is there really nothing more clever we can do? > > > > You get this part in the IBRS/microcode solution too. The IBRS MSR > > doesn't catch everything; you still need to stuff the RSB in very > > similar places (and/or use the IBPB MSR in some). >=20 > So I was really hoping that in places like context switching etc, we'd > be able to instead effectively kill off any exploits by clearing > registers. >=20 > That should make it pretty damn hard to then find a matching "gadget" > that actually does anything interesting/powerful. >=20 > Together with Spectre already being pretty hard to take advantage of, > and the eBPF people making those user-proivided gadgets inaccessible, > it really should be a pretty powerful fix. Hm... on a context switch you're reloading the registers that were in the other saved context. If the attacker does something they know will go into a deep call stack and then sleep, and they pollute the BTB for every 'ret' instruction in that stack=C2=B9, then the registers at the time a 'ret' goes AWOL are not necessarily something you can 'clear' at the time of the context switch. And yes, a lot of this v2 stuff down past the sys_call_table branch really *is* pretty damn hard already, especially the 'ret' based ones. =C2=B9 perhaps they do the BTB-pollution in their other CPU-hogging thread =C2=A0 that runs on the same CPU when the attack thread is sleeping? I'm no= t =C2=A0 really trying to actually write an attack here, just thinking out =C2=A0 loud and being uncertain that you've actually proved there *isn't* =C2=A0 one :) --=-WMUDCYvaMVoWGQSaqWxk 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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