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 B9396C433E1 for ; Fri, 31 Jul 2020 18:32:11 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 2E1F722B3F for ; Fri, 31 Jul 2020 18:32:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2E1F722B3F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19509-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 5933 invoked by uid 550); 31 Jul 2020 18:32:04 -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 5913 invoked from network); 31 Jul 2020 18:32:03 -0000 Date: Fri, 31 Jul 2020 19:31:46 +0100 From: Mark Rutland To: "Madhavan T. Venkataraman" Cc: Andy Lutomirski , 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 Message-ID: <20200731183146.GD67415@C02TD0UTHF1T.local> References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <6540b4b7-3f70-adbf-c922-43886599713a@linux.microsoft.com> <46a1adef-65f0-bd5e-0b17-54856fb7e7ee@linux.microsoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46a1adef-65f0-bd5e-0b17-54856fb7e7ee@linux.microsoft.com> On Fri, Jul 31, 2020 at 12:13:49PM -0500, Madhavan T. Venkataraman wrote: > On 7/30/20 3:54 PM, Andy Lutomirski wrote: > > On Thu, Jul 30, 2020 at 7:24 AM Madhavan T. Venkataraman > > wrote: > Dealing with multiple architectures > ----------------------------------------------- > > One good reason to use trampfd is multiple architecture support. The > trampoline table in a code page approach is neat. I don't deny that at > all. But my question is - can it be used in all cases? > > It requires PC-relative data references. I have not worked on all architectures. > So, I need to study this. But do all ISAs support PC-relative data references? Not all do, but pretty much any recent ISA will as it's a practical necessity for fast position-independent code. > Even in an ISA that supports it, there would be a maximum supported offset > from the current PC that can be reached for a data reference. That maximum > needs to be at least the size of a base page in the architecture. This is because > the code page and the data page need to be separate for security reasons. > Do all ISAs support a sufficiently large offset? ISAs with pc-relative addessing can usually generate PC-relative addresses into a GPR, from which they can apply an arbitrarily large offset. > When the kernel generates the code for a trampoline, it can hard code data values > in the generated code itself so it does not need PC-relative data referencing. > > And, for ISAs that do support the large offset, we do have to implement and > maintain the code page stuff for different ISAs for each application and library > if we did not use trampfd. Trampoline code is architecture specific today, so I don't see that as a major issue. Common structural bits can probably be shared even if the specifid machine code cannot. [...] > Security > ----------- > > With the user level trampoline table approach, the data part of the trampoline table > can be hacked by an attacker if an application has a vulnerability. Specifically, the > target PC can be altered to some arbitrary location. Trampfd implements an > "Allowed PCS" context. In the libffi changes, I have created a read-only array of > all ABI handlers used in closures for each architecture. This read-only array > can be used to restrict the PC values for libffi trampolines to prevent hacking. > > To generalize, we can implement security rules/features if the trampoline > object is in the kernel. I don't follow this argument. If it's possible to statically define that in the kernel, it's also possible to do that in userspace without any new kernel support. [...] > Trampfd is a framework that can be used to implement multiple things. May be, > a few of those things can also be implemented in user land itself. But I think having > just one mechanism to execute dynamic code objects is preferable to having > multiple mechanisms not standardized across all applications. In abstract, having a common interface sounds nice, but in practice elements of this are always architecture-specific (e.g. interactiosn with HW CFI), and that common interface can result in more pain as it doesn't fit naturally into the context that ISAs were designed for (e.g. where control-flow instructions are extended with new semantics). It also meass that you can't share the rough approach across OSs which do not implement an identical mechanism, so for code abstracting by ISA first, then by platform/ABI, there isn't much saving. Thanks, Mark.