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=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 C9191C433E1 for ; Mon, 24 Aug 2020 15:07:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AA4E3207CD for ; Mon, 24 Aug 2020 15:07:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="lB8woOsv" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726986AbgHXPHJ (ORCPT ); Mon, 24 Aug 2020 11:07:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39430 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726650AbgHXPFb (ORCPT ); Mon, 24 Aug 2020 11:05:31 -0400 Received: from mail-wm1-x32c.google.com (mail-wm1-x32c.google.com [IPv6:2a00:1450:4864:20::32c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 78682C0617A9 for ; Mon, 24 Aug 2020 08:05:11 -0700 (PDT) Received: by mail-wm1-x32c.google.com with SMTP id 83so8661634wme.4 for ; Mon, 24 Aug 2020 08:05:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=4Z9CfRsQ0JPDJK+OdpY56vYSFlw0MvtWCYo+D9zzr0o=; b=lB8woOsvvYhCqqAhBrrOI4RyghVGWXTQFIz7QP/geKRTgxJ5Ydbnf0ULekEb2fnx6E LIx5ZXOroEf5QkkzXGFUuatWpWzKU8BXTOBLol8vcK0GyEzvqLzwcxlZnouQiPxO3vQ9 YGWWtTbmSO/FTFZTk4MxFuHaMnbSLcTjsA1ixnxCiRwMl/N9Pfkdkvi4wlAN5mU6s9lB gfGU+rWbIdYtgAAD952agaykme1dVHqFB5Gh+HyTuc0ShwRZSGX5SVSVxfFojiNsdqwW dbYPzTJBm3+Wps1nFdfCfuJTK4Z1wFTBwlBSW4yF/FROXrF4h55RK2I5C2uAGtuCIbGW 1k+A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=4Z9CfRsQ0JPDJK+OdpY56vYSFlw0MvtWCYo+D9zzr0o=; b=Y5Z2XUBiIy6BDMw7VJ03CRmWmKkYiQw4M1/A2ldPEt7ogyjSsNjU9DmeE3vRSKNMxe m/1eIJ+sN57lA9xJ9y8Bn4BDHP9QRZf27s5sJB/lJgswPXI/lZudW8NC9YN7e1gXz3q9 UFx8otVpmN+crIVXqRoDlvF4KDJ+z9eivaLn3WREjaNiiYMNbw7LPS07NTPGjYYTHXP+ KOa5FhQexo0Tpz4ng+uoTVmcroNPXqU13XGVVxsQOPCDHJqsyL75sNghheioUMDNvEfI +PKYVEDv5tAlQ4jihQSg3A6FAvtSw6ehte7inyPRrER71+4Zv+LMbHb6xT6gPG1WRp8c JBLA== X-Gm-Message-State: AOAM532yo6frPGV8wHyVNBFzu2IUddxjtwbCGpiZoiXfZGNZMfA/5ePH pvHXoM/TztzwSieGhaSJe9BtAA== X-Google-Smtp-Source: ABdhPJz3KjGHHQA/f94TebnygpE4VIUsNqkOfejclgeosK32BVVg2F+4exTc1VOTgF6WyK7QdPxObw== X-Received: by 2002:a7b:cd88:: with SMTP id y8mr5875966wmj.14.1598281509685; Mon, 24 Aug 2020 08:05:09 -0700 (PDT) Received: from google.com ([2a00:79e0:42:204:1ea0:b8ff:fe80:839]) by smtp.gmail.com with ESMTPSA id y13sm14526834wrn.48.2020.08.24.08.05.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Aug 2020 08:05:08 -0700 (PDT) Date: Mon, 24 Aug 2020 17:05:04 +0200 From: Brendan Jackman To: Peter Zijlstra Cc: Kees Cook , Brendan Jackman , linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-security-module@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , James Morris , pjt@google.com, Jann Horn , rafael.j.wysocki@intel.com, thgarnie@chromium.org, KP Singh , paul.renauld.epfl@gmail.com Subject: Re: [RFC] security: replace indirect calls with static calls Message-ID: <20200824150504.GA575149@google.com> References: <20200820164753.3256899-1-jackmanb@chromium.org> <202008201435.97CF8296@keescook> <20200824143344.GB3982@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200824143344.GB3982@worktop.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 24, 2020 at 04:33:44PM +0200, Peter Zijlstra wrote: > On Mon, Aug 24, 2020 at 04:09:09PM +0200, Brendan Jackman wrote: > > > > > Why this trick with a switch statement? The table of static call is defined > > > > at compile time. The number of hook callbacks that will be defined is > > > > unknown at that time, and the table cannot be resized at runtime. Static > > > > calls do not define a conditional execution for a non-void function, so the > > > > executed slots must be non-empty. With this use of the table and the > > > > switch, it is possible to jump directly to the first used slot and execute > > > > all of the slots after. This essentially makes the entry point of the table > > > > dynamic. Instead, it would also be possible to start from 0 and break after > > > > the final populated slot, but that would require an additional conditional > > > > after each slot. > > > > > > Instead of just "NOP", having the static branches perform a jump would > > > solve this pretty cleanly, yes? Something like: > > > > > > ret = DEFAULT_RET; > > > > > > ret = A(args); <--- direct call, no retpoline > > > if ret != 0: > > > goto out; > > > > > > ret = B(args); <--- direct call, no retpoline > > > if ret != 0: > > > goto out; > > > > > > goto out; > > > if ret != 0: > > > goto out; > > > > > > out: > > > return ret; > > > > Hmm yeah that's a cool idea. This would either need to be implemented > > with custom code-modification logic for the LSM hooks, or we'd need to > > think of a way to express it in a sensible addition to the static_call > > API. I do wonder if the latter could take the form of a generic system > > for arrays of static calls. > > So you basically want something like: > > if (A[0] && (ret = static_call(A[0])(...))) > return ret; > > if (A[1] && (ret = static_call(A[1])(...))) > return ret; > > .... > > return ret; > > Right? The problem with static_call_cond() is that we don't know what to > do with the return value when !func, which is why it's limited to void > return type. > > You can however construct something like the above with a combination of > static_branch() and static_call() though. It'll not be pretty, but it > ought to work: > > if (static_branch_likely(A[0].key)) { > ret = static_call(A[0].call)(...); > if (ret) > return ret; > } > > ... > > return ret; > Right. That's actually exactly what Paul's first implementation looked like for call_int_hook. But we thought the switch thing was easier to understand. > > > It would also need to handle the fact that IIUC at the moment the last > > static_call may be a tail call, so we'd be patching an existing jump > > into a jump to a different target, I don't know if we can do that > > atomically. > > Of course we can, the static_call() series supports tail-calls just > fine. In fact, patching jumps is far easier, it was patching call that > was the real problem because it mucks about with the stack. > OK great. I had a vague apprehension that we could only patch to or from a NOP.