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=-0.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 6DB78C04AB1 for ; Thu, 9 May 2019 14:42:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 448382053B for ; Thu, 9 May 2019 14:42:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726831AbfEIOm5 (ORCPT ); Thu, 9 May 2019 10:42:57 -0400 Received: from ozlabs.org ([203.11.71.1]:47891 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726251AbfEIOm5 (ORCPT ); Thu, 9 May 2019 10:42:57 -0400 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 450GKT2bqhz9s00; Fri, 10 May 2019 00:42:53 +1000 (AEST) From: Michael Ellerman To: Petr Mladek Cc: linuxppc-dev@lists.ozlabs.org, Russell Currey , Christophe Leroy , Stephen Rothwell , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: Crashes in linux-next on powerpc with CONFIG_PPC_KUAP and CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG In-Reply-To: <20190509092942.ei4myfzt5dczuptj@pathway.suse.cz> References: <87k1f2wc04.fsf@concordia.ellerman.id.au> <20190509092942.ei4myfzt5dczuptj@pathway.suse.cz> Date: Fri, 10 May 2019 00:42:52 +1000 Message-ID: <87woizvgcz.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Petr Mladek writes: > On Wed 2019-05-08 00:54:51, Michael Ellerman wrote: >> Hi folks, >> >> Just an FYI in case anyone else is seeing crashes very early in boot in >> linux-next with the above config options. >> >> The problem is the combination of some new code called via printk(), >> check_pointer() which calls probe_kernel_read(). That then calls >> allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early >> (before we've patched features). With the JUMP_LABEL debug enabled that >> causes us to call printk() & dump_stack() and we end up recursing and >> overflowing the stack. > > Sigh, the check_pointer() stuff is in Linus's tree now, see > the commit 3e5903eb9cff707301712 ("vsprintf: Prevent crash when > dereferencing invalid pointers"). No worries. >> Because it happens so early you don't get any output, just an apparently >> dead system. >> >> The stack trace (which you don't see) is something like: >> >> ... >> dump_stack+0xdc >> probe_kernel_read+0x1a4 >> check_pointer+0x58 >> string+0x3c >> vsnprintf+0x1bc >> vscnprintf+0x20 >> printk_safe_log_store+0x7c >> printk+0x40 >> dump_stack_print_info+0xbc >> dump_stack+0x8 >> probe_kernel_read+0x1a4 >> probe_kernel_read+0x19c >> check_pointer+0x58 >> string+0x3c >> vsnprintf+0x1bc >> vscnprintf+0x20 >> vprintk_store+0x6c >> vprintk_emit+0xec >> vprintk_func+0xd4 >> printk+0x40 >> cpufeatures_process_feature+0xc8 >> scan_cpufeatures_subnodes+0x380 >> of_scan_flat_dt_subnodes+0xb4 >> dt_cpu_ftrs_scan_callback+0x158 >> of_scan_flat_dt+0xf0 >> dt_cpu_ftrs_scan+0x3c >> early_init_devtree+0x360 >> early_setup+0x9c >> >> >> The simple fix is to use early_mmu_has_feature() in allow_user_access(), >> but we'd rather not do that because it penalises all >> copy_to/from_users() for the life of the system with the cost of the >> runtime check vs the jump label. The irony is probe_kernel_read() >> shouldn't be allowing user access at all, because we're reading the >> kernel not userspace. > > I have tried to find a lightweight way for a safe reading of unknown > kernel pointer. But I have not succeeded so far. I see only variants > with user access. The user access is handled in arch-specific code > and I do not see any variant without it. > > I am not sure on which level it should get fixed. I sent a fix in powerpc code (sorry might have forgot to Cc you): https://patchwork.ozlabs.org/patch/1097015/ I've merged that into the powerpc tree. I think it's too subtle for us to have an ordering requirement that deep in the user copy code, it was just a matter of time before it caused a problem, you were just unlucky it was your patch that did :) We'll eventually switch it back to using a jump label but make it safe to call early in boot before we've detected features. > Could you please send it to lkml to get a wider audience? I see you also sent a fix, that looks like a safe default to me. But as I said I'm happy with the powerpc fix, so there's no requirement from us that your fix get merged. cheers