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, URIBL_BLOCKED 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 1F4BBC433E1 for ; Mon, 3 Aug 2020 08:08:44 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 7DCD82065E for ; Mon, 3 Aug 2020 08:08:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7DCD82065E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=ACULAB.COM Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19529-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 11628 invoked by uid 550); 3 Aug 2020 08:08:36 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 11601 invoked from network); 3 Aug 2020 08:08:36 -0000 X-MC-Unique: PHrlPE41MKu5ed8gAMWPYg-1 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: quoted-printable From: Pavel Machek > Sent: 02 August 2020 12:56 > Hi! >=20 > > > This is quite clever, but now I???m wondering just how much kernel he= lp > > > 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: > > =09mov=09$0, scratch_reg; jmp trampoline_handler >=20 > I believe you could do with mov %pc, scratch_reg; jmp ... >=20 > 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. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)