From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D6EEC433DF for ; Mon, 3 Aug 2020 08:08:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC8AC20678 for ; Mon, 3 Aug 2020 08:08:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725958AbgHCII1 convert rfc822-to-8bit (ORCPT ); Mon, 3 Aug 2020 04:08:27 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:21279 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725907AbgHCII0 (ORCPT ); Mon, 3 Aug 2020 04:08:26 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-111-PHrlPE41MKu5ed8gAMWPYg-1; Mon, 03 Aug 2020 09:08:22 +0100 X-MC-Unique: PHrlPE41MKu5ed8gAMWPYg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 3 Aug 2020 09:08:21 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 3 Aug 2020 09:08:21 +0100 From: David Laight To: 'Pavel Machek' CC: 'Andy Lutomirski' , "madvenka@linux.microsoft.com" , Kernel Hardening , Linux API , linux-arm-kernel , Linux FS Devel , linux-integrity , LKML , "LSM List" , Oleg Nesterov , X86 ML Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Topic: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Index: AQHWZQT/T+e4gDrzGEmP/30MMvDTCqkgFteggASWI4CAAWJTUA== Date: Mon, 3 Aug 2020 08:08:21 +0000 Message-ID: References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <20200802115600.GB1162@bug> In-Reply-To: <20200802115600.GB1162@bug> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: Archived-At: List-Archive: List-Post: From: Pavel Machek > Sent: 02 August 2020 12:56 > Hi! > > > > This is quite clever, but now I???m wondering just how much kernel help > > > is really needed. In your series, the trampoline is an non-executable > > > page. I can think of at least two alternative approaches, and I'd > > > like to know the pros and cons. > > > > > > 1. Entirely userspace: a return trampoline would be something like: > > > > > > 1: > > > pushq %rax > > > pushq %rbc > > > pushq %rcx > > > ... > > > pushq %r15 > > > movq %rsp, %rdi # pointer to saved regs > > > leaq 1b(%rip), %rsi # pointer to the trampoline itself > > > callq trampoline_handler # see below > > > > For nested calls (where the trampoline needs to pass the > > original stack frame to the nested function) I think you > > just need a page full of: > > mov $0, scratch_reg; jmp trampoline_handler > > I believe you could do with mov %pc, scratch_reg; jmp ... > > That has advantage of being able to share single physical > page across multiple virtual pages... A lot of architecture don't let you copy %pc that way so you would have to use 'call' - but that trashes the return address cache. It also needs the trampoline handler to know the addresses of the trampolines. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)