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=-14.8 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 8655AC433E0 for ; Tue, 23 Mar 2021 16:54:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C787619C3 for ; Tue, 23 Mar 2021 16:54:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230416AbhCWQxj (ORCPT ); Tue, 23 Mar 2021 12:53:39 -0400 Received: from linux.microsoft.com ([13.77.154.182]:55348 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231842AbhCWQxG (ORCPT ); Tue, 23 Mar 2021 12:53:06 -0400 Received: from [192.168.254.32] (unknown [47.187.194.202]) by linux.microsoft.com (Postfix) with ESMTPSA id 3C43E20B5680; Tue, 23 Mar 2021 09:53:05 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 3C43E20B5680 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1616518385; bh=bsWnf5PwIqc8V0tuB6C6T1DpJkJTvse0YHx93Jrj8Xg=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=Bkm0j+9ZRXt0B8Tn2aSIRanJbDitVzQx4He1n2vaiw/cjk+mhWR23xruB+O7BjPYw Swcynk9xosQ+mND+C0Az/N1ay2W54jPgIfK0XAYxmHRfYK6XvuU/GKFWZ+FdHYQgEC uoC8Bo5zulY5pM92BSr5N3ZxKFIP1ad8riDSJdiM= Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable To: Mark Rutland Cc: broonie@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <5997dfe8d261a3a543667b83c902883c1e4bd270> <20210315165800.5948-1-madvenka@linux.microsoft.com> <20210315165800.5948-6-madvenka@linux.microsoft.com> <20210323105118.GE95840@C02TD0UTHF1T.local> <2167f3c5-e7d0-40c8-99e3-ae89ceb2d60e@linux.microsoft.com> <20210323133611.GB98545@C02TD0UTHF1T.local> <20210323145734.GD98545@C02TD0UTHF1T.local> <20210323164801.GE98545@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: Date: Tue, 23 Mar 2021 11:53:04 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210323164801.GE98545@C02TD0UTHF1T.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/23/21 11:48 AM, Mark Rutland wrote: > On Tue, Mar 23, 2021 at 10:26:50AM -0500, Madhavan T. Venkataraman wrote: >> On 3/23/21 9:57 AM, Mark Rutland wrote: >> Thanks for explaining the nesting. It is now clear to me. > > No problem! > >> So, my next question is - can we define a practical limit for the >> nesting so that any nesting beyond that is fatal? The reason I ask is >> - if there is a max, then we can allocate an array of stack frames out >> of band for the special frames so they are not part of the stack and >> will not likely get corrupted. > > I suspect we can't define such a fatal limit without introducing a local > DoS vector on some otherwise legitimate workload, and I fear this will > further complicate the entry/exit logic, so I'd prefer to avoid > introducing a new limit. > I suspected as much. But I thought I will ask anyway. > What exactly do you mean by a "special frame", and why do those need > additional protection over regular frame records? > Special frame just means pt_regs->stackframe that is used for exceptions. No additional protection is needed. I just meant that since they are out of band, we can reliably tell that there are exceptions without examining the stack. That is all. >> Also, we don't have to do any special detection. If the number of out >> of band frames used is one or more then we have exceptions and the >> stack trace is unreliable. > > What is expected to protect against? > It is not a protection thing. I just wanted a reliable way to tell that there is an exception without having to unwind the stack up to the exception frame. That is all. Thanks. Madhavan