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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 DAF57C433E0 for ; Mon, 18 May 2020 13:32:42 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A4E6C20756 for ; Mon, 18 May 2020 13:32:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="WMWDDqge" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A4E6C20756 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=wGMk3sAzendnUdmn5m7vsJ1Nj6Pe40A1M276s26WoHo=; b=WMWDDqgemCbbE6 +7b8fMB3SyDtc9k4q8UOf85lsqo8GAMaoa9VMj5soEcl3cXHWR8ZCfvbYwvAPcCybQfwEw7Gy0hU2 ShneUSlTgpQPpNfRcjQUyMYUoD7tZ7hj2AK+pPhsq+HyFCxZEHxXiakf06H0j5RKMbyY3bWOAOwP+ 6oCNULThfJmCLb2f1jI1bIqFeBVj2DByI6rg8IMV9wFDc+Ie4O6vbrUKKTPD3ih6LY/hUqqDRSMj6 pLJ1vA2N9LMdWnN3ZS/qsL/ijFZK5lvo/PdfG24qRKMbtu6yBaqbxx9lUoSkGWfgkAYl7TN6O779G G3v0/wZrpYn2Yrii9QZw==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jafsn-00074u-Im; Mon, 18 May 2020 13:32:41 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jafsk-00073x-7y for linux-arm-kernel@lists.infradead.org; Mon, 18 May 2020 13:32:39 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4178C101E; Mon, 18 May 2020 06:32:37 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.29.34]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D82EF3F305; Mon, 18 May 2020 06:32:34 -0700 (PDT) Date: Mon, 18 May 2020 14:32:31 +0100 From: Mark Rutland To: Will Deacon Subject: Re: [PATCH 4/6] scs: Move scs_overflow_check() out of architecture code Message-ID: <20200518133231.GC2787@C02TD0UTHF1T.local> References: <20200515172756.27185-1-will@kernel.org> <20200515172756.27185-5-will@kernel.org> <20200518121210.GD1957@C02TD0UTHF1T.local> <20200518132346.GD32394@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200518132346.GD32394@willie-the-truck> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200518_063238_371676_02470379 X-CRM114-Status: GOOD ( 29.13 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kees Cook , Jann Horn , Peter Zijlstra , Catalin Marinas , linux-kernel@vger.kernel.org, Mark Rutland , Sami Tolvanen , kernel-team@android.com, Ard Biesheuvel , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, May 18, 2020 at 02:23:47PM +0100, Will Deacon wrote: > On Mon, May 18, 2020 at 01:12:10PM +0100, Mark Rutland wrote: > > On Fri, May 15, 2020 at 06:27:54PM +0100, Will Deacon wrote: > > > There is nothing architecture-specific about scs_overflow_check() as > > > it's just a trivial wrapper around scs_corrupted(). > > > > > > For parity with task_stack_end_corrupted(), rename scs_corrupted() to > > > task_scs_end_corrupted() and call it from schedule_debug() when > > > CONFIG_SCHED_STACK_END_CHECK_is enabled. Finally, remove the unused > > > scs_overflow_check() function entirely. > > > > > > This has absolutely no impact on architectures that do not support SCS > > > (currently arm64 only). > > > > > > Signed-off-by: Will Deacon > > > > Pulling this out of arch code seems sane to me, and the arch-specific > > chanes look sound. However, I have a concern with the changes within the > > scheduler context-switch. > > > > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > > index a35d3318492c..56be4cbf771f 100644 > > > --- a/arch/arm64/kernel/process.c > > > +++ b/arch/arm64/kernel/process.c > > > @@ -52,7 +52,6 @@ > > > #include > > > #include > > > #include > > > -#include > > > #include > > > > > > #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK) > > > @@ -516,7 +515,6 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev, > > > entry_task_switch(next); > > > uao_thread_switch(next); > > > ssbs_thread_switch(next); > > > - scs_overflow_check(next); > > > > Prior to this patch, we'd never switch to a task whose SCS had already > > been corrupted. > > > > With this patch, we only check that when switching away from a task, and > > only when CONFIG_SCHED_STACK_END_CHECK is selected, which at first > > glance seems to weaken that. > > Yes, ignoring vmap'd stacks, this patch brings the SCS checking in-line with > the main stack checking when CONFIG_SCHED_STACK_END_CHECK=y. > > > Arguably: > > > > * If the next task's SCS was corrupted by that task while it was > > running, we had already lost at that point. > > With this change, we'll at least catch this one sooner, and that might be > useful if a bug has caused us to overflow the SCS but not the main stack. Sure, but only if CONFIG_SCHED_STACK_END_CHECK is selected. > > * If the next task's SCS was corrupted by another task, then that could > > also happen immediately after the check (though timing to avoid the > > check but affect the process could be harder). > > We're only checking the magic end value, so the cross-task case is basically > if you overrun your own SCS as above, but then continue to overrun entire > SCSs for other tasks as well. It's probably not very useful in that case. > > > ... and a VMAP'd SCS would be much nicer in this regard. > > > > Do we think this is weakening the check, or do we think it wasn't all > > that helpful to begin with? > > I see it as a debug check to catch SCS overflow, rather than a hardening > feature, and I agree that using something like vmap stack for the SCS would > be better because we could have a guard page instead. Fair enough. Could we put something into the commit message that more explicitly calls out debug-not-hardening? I agree that under that model this patch looks fine, and with something to that effect: Reviewed-by: Mark Rutland Mark. > This is something I would like to revisit, but we need more > information from Sami about why Android rejected the larger allocation > size, since I don't think there's an awful lot of point merging this > series if Android doesn't pick it up. Indeed. I'd certainly prefer the robustness of a VMAP'd SCS if we can do that. Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel