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=-13.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL 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 981B1C433E1 for ; Tue, 28 Jul 2020 16:58:08 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id E61CB2053B for ; Tue, 28 Jul 2020 16:58:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="WcBQe+fl" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E61CB2053B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19476-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 17716 invoked by uid 550); 28 Jul 2020 16:57:56 -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 7460 invoked from network); 28 Jul 2020 16:49:23 -0000 DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 00A5820B4908 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1595954951; bh=Wp9lMVW/v/maF95Uhld2YeppHP7BkSsiQSbIpc2iGG8=; h=Subject:To:References:From:Date:In-Reply-To:From; b=WcBQe+flSDX55q/ewMf9Af18CEdGVpPWvr4lx2WIFz5MF35ysP3AWSLhovuuIPJcA R3zbmGw3qgHmAG6CF4LvqSk2K2IKdZOIS5Qx4ZwAsGDJLulERBJPTE+dh47a06ZwIC G6ZCs/L/b81Zc5VW94CZcASUxH6PlX38Z+nqyTKc= Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor To: Casey Schaufler , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, oleg@redhat.com, x86@kernel.org References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <3fd22f92-7f45-1b0f-e4fe-857f3bceedd0@schaufler-ca.com> From: "Madhavan T. Venkataraman" Message-ID: <80dd0421-062b-bfaa-395a-e52b169acea4@linux.microsoft.com> Date: Tue, 28 Jul 2020 11:49:10 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <3fd22f92-7f45-1b0f-e4fe-857f3bceedd0@schaufler-ca.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US Thanks. On 7/28/20 11:05 AM, Casey Schaufler wrote: >> In this solution, the kernel recognizes certain sequences of instructions >> as "well-known" trampolines. When such a trampoline is executed, a page >> fault happens because the trampoline page does not have execute permission. >> The kernel recognizes the trampoline and emulates it. Basically, the >> kernel does the work of the trampoline on behalf of the application. > What prevents a malicious process from using the "well-known" trampoline > to its own purposes? I expect it is obvious, but I'm not seeing it. Old > eyes, I suppose. You are quite right. As I note below, the attack surface is the buffer that contains the trampoline code. Since the kernel does check the instruction sequence, the sequence cannot be changed by a hacker. But the hacker can presumably change the register values and redirect the PC to his desired location. The assumption with trampoline emulation is that the system will have security settings that will prevent pages from having both write and execute permissions. So, a hacker cannot load his own code in a page and redirect the PC to it and execute his own code. But he can probably set the PC to point to arbitrary locations. For instance, jump to the middle of a C library function. > >> Here, the attack surface is the buffer that contains the trampoline. >> The attack surface is narrower than before. A hacker may still be able to >> modify what gets loaded in the registers or modify the target PC to point >> to arbitrary locations. ... >> Work that is pending >> -------------------- >> >> - I am working on implementing an SELinux setting called "exectramp" >> similar to "execmem" to allow the use of trampfd on a per application >> basis. > You could make a separate LSM to do these checks instead of limiting > it to SELinux. Your use case, your call, of course. OK. I will research this. Madhavan