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=-2.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT 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 B3177C04AB1 for ; Thu, 9 May 2019 09:29:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7F5FB21479 for ; Thu, 9 May 2019 09:29:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726744AbfEIJ3q (ORCPT ); Thu, 9 May 2019 05:29:46 -0400 Received: from mx2.suse.de ([195.135.220.15]:58592 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725821AbfEIJ3o (ORCPT ); Thu, 9 May 2019 05:29:44 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 966AAAB42; Thu, 9 May 2019 09:29:43 +0000 (UTC) Date: Thu, 9 May 2019 11:29:42 +0200 From: Petr Mladek To: Michael Ellerman 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 Message-ID: <20190509092942.ei4myfzt5dczuptj@pathway.suse.cz> References: <87k1f2wc04.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k1f2wc04.fsf@concordia.ellerman.id.au> User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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"). > 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. Could you please send it to lkml to get a wider audience? Best Regards, Petr > For now if you're hitting it just turn off > CONFIG_PPC_KUAP and/or CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG. > > cheers