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 D3C46C433E0 for ; Tue, 23 Mar 2021 15:23:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 97532619BB for ; Tue, 23 Mar 2021 15:23:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232553AbhCWPW4 (ORCPT ); Tue, 23 Mar 2021 11:22:56 -0400 Received: from linux.microsoft.com ([13.77.154.182]:43954 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232314AbhCWPWl (ORCPT ); Tue, 23 Mar 2021 11:22:41 -0400 Received: from [192.168.254.32] (unknown [47.187.194.202]) by linux.microsoft.com (Postfix) with ESMTPSA id 99E9120B5680; Tue, 23 Mar 2021 08:22:40 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 99E9120B5680 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1616512961; bh=EI1uKbfraV1cG4bEEEQBV8q/N2mbFDxBXYmbzNTRNhU=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=YjkHvocvBFcfhWo2dnhR2Sg8VXZPlSInO814iKW9ptxTWzZH2AvuzS3rdBcHdW43A 6XdYqagXUzlT1Lb8pitoPi+cS+65my457qgifj11SKajo0y6+sJsjegl4KIuU0b/Mg MsTCpsG2qw8FZ+gc7ax1b1YD3iBD1xO3bQzwJF0Q= Subject: Re: [RFC PATCH v2 4/8] arm64: Detect an EL1 exception 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> <20210315165800.5948-5-madvenka@linux.microsoft.com> <20210323104251.GD95840@C02TD0UTHF1T.local> <20210323130425.GA98545@C02TD0UTHF1T.local> <20210323143345.GC98545@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: Date: Tue, 23 Mar 2021 10:22:39 -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: <20210323143345.GC98545@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 9:33 AM, Mark Rutland wrote: > On Tue, Mar 23, 2021 at 08:31:50AM -0500, Madhavan T. Venkataraman wrote: >> On 3/23/21 8:04 AM, Mark Rutland wrote: >>> On Tue, Mar 23, 2021 at 07:46:10AM -0500, Madhavan T. Venkataraman wrote: >>>> On 3/23/21 5:42 AM, Mark Rutland wrote: >>>>> On Mon, Mar 15, 2021 at 11:57:56AM -0500, madvenka@linux.microsoft.com wrote: >>>>>> From: "Madhavan T. Venkataraman" >>>>>> >>>>>> EL1 exceptions can happen on any instruction including instructions in >>>>>> the frame pointer prolog or epilog. Depending on where exactly they happen, >>>>>> they could render the stack trace unreliable. >>>>>> >>>>>> If an EL1 exception frame is found on the stack, mark the stack trace as >>>>>> unreliable. >>>>>> >>>>>> Now, the EL1 exception frame is not at any well-known offset on the stack. >>>>>> It can be anywhere on the stack. In order to properly detect an EL1 >>>>>> exception frame the following checks must be done: >>>>>> >>>>>> - The frame type must be EL1_FRAME. >>>>>> >>>>>> - When the register state is saved in the EL1 pt_regs, the frame >>>>>> pointer x29 is saved in pt_regs->regs[29] and the return PC >>>>>> is saved in pt_regs->pc. These must match with the current >>>>>> frame. >>>>> >>>>> Before you can do this, you need to reliably identify that you have a >>>>> pt_regs on the stack, but this patch uses a heuristic, which is not >>>>> reliable. >>>>> >>>>> However, instead you can identify whether you're trying to unwind >>>>> through one of the EL1 entry functions, which tells you the same thing >>>>> without even having to look at the pt_regs. >>>>> >>>>> We can do that based on the entry functions all being in .entry.text, >>>>> which we could further sub-divide to split the EL0 and EL1 entry >>>>> functions. >>>> >>>> Yes. I will check the entry functions. But I still think that we should >>>> not rely on just one check. The additional checks will make it robust. >>>> So, I suggest that the return address be checked first. If that passes, >>>> then we can be reasonably sure that there are pt_regs. Then, check >>>> the other things in pt_regs. >>> >>> What do you think this will catch? >> >> I am not sure that I have an exact example to mention here. But I will attempt >> one. Let us say that a task has called arch_stack_walk() in the recent past. >> The unwinder may have copied a stack frame onto some location in the stack >> with one of the return addresses we check. Let us assume that there is some >> stack corruption that makes a frame pointer point to that exact record. Now, >> we will get a match on the return address on the next unwind. > > I don't see how this is material to the pt_regs case, as either: > > * When the unwinder considers this frame, it appears to be in the middle > of an EL1 entry function, and the unwinder must mark the unwinding as > unreliable regardless of the contents of any regs (so there's no need > to look at the regs). > > * When the unwinder considers this frame, it does not appear to be in > the middle of an EL1 entry function, so the unwinder does not think > there are any regs to consider, and so we cannot detect this case. > > ... unless I've misunderstood the example? > > There's a general problem that it's possible to corrupt any portion of > the chain to skip records, e.g. > > A -> B -> C -> D -> E -> F -> G -> H -> [final] > > ... could get corrupted to: > > A -> B -> D -> H -> [final] > > ... regardless of whether C/E/F/G had associated pt_regs. AFAICT there's > no good way to catch this generally unless we have additional metadata > to check the unwinding against. > > The likelihood of this happening without triggering other checks is > vanishingly low, and as we don't have a reliable mechanism for detecting > this, I don't think it's worthwhile attempting to do so. > > If and when we try to unwind across EL1 exception boundaries, the > potential mismatch between the frame record and regs will be more > significant, and I agree at that point thisd will need more thought. > >> Pardon me if the example is somewhat crude. My point is that it is >> highly unlikely but not impossible for the return address to be on the >> stack and for the unwinder to get an unfortunate match. > > I agree that this is possible in theory, but as above I don't think this > is a practical concern. > OK. What you say makes sense. Thanks. Madhavan