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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 5DB36C4CED1 for ; Fri, 4 Oct 2019 09:11:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3F846222C0 for ; Fri, 4 Oct 2019 09:11:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730992AbfJDJLq (ORCPT ); Fri, 4 Oct 2019 05:11:46 -0400 Received: from mx2.suse.de ([195.135.220.15]:53964 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730921AbfJDJLq (ORCPT ); Fri, 4 Oct 2019 05:11:46 -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 9FDC6AEFB; Fri, 4 Oct 2019 09:11:43 +0000 (UTC) Date: Fri, 4 Oct 2019 11:11:42 +0200 From: Petr Mladek To: Will Deacon Cc: Kees Cook , Russell King , Sergey Senozhatsky , Steven Rostedt , Feng Tang , Andrew Morton , Greg Kroah-Hartman , linux-arm-kernel@lists.infradead.org, Ingo Molnar , linux-kernel@vger.kernel.org, stable@vger.kernel.org, contact@xogium.me Subject: Re: [PATCH] panic: Ensure preemption is disabled during panic() Message-ID: <20191004091142.57iylai22aqpu6lu@pathway.suse.cz> References: <20191002123538.22609-1-will@kernel.org> <201910021355.E578D2FFAF@keescook> <20191003205633.w26geqhq67u4ysit@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191003205633.w26geqhq67u4ysit@willie-the-truck> 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 Thu 2019-10-03 21:56:34, Will Deacon wrote: > Hi Kees, > > On Wed, Oct 02, 2019 at 01:58:46PM -0700, Kees Cook wrote: > > On Wed, Oct 02, 2019 at 01:35:38PM +0100, Will Deacon wrote: > > > Calling 'panic()' on a kernel with CONFIG_PREEMPT=y can leave the > > > calling CPU in an infinite loop, but with interrupts and preemption > > > enabled. From this state, userspace can continue to be scheduled, > > > despite the system being "dead" as far as the kernel is concerned. This > > > is easily reproducible on arm64 when booting with "nosmp" on the command > > > line; a couple of shell scripts print out a periodic "Ping" message > > > whilst another triggers a crash by writing to /proc/sysrq-trigger: > > > > > > | sysrq: Trigger a crash > > > | Kernel panic - not syncing: sysrq triggered crash > > > | CPU: 0 PID: 1 Comm: init Not tainted 5.2.15 #1 > > > | Hardware name: linux,dummy-virt (DT) > > > | Call trace: > > > | dump_backtrace+0x0/0x148 > > > | show_stack+0x14/0x20 > > > | dump_stack+0xa0/0xc4 > > > | panic+0x140/0x32c > > > | sysrq_handle_reboot+0x0/0x20 > > > | __handle_sysrq+0x124/0x190 > > > | write_sysrq_trigger+0x64/0x88 > > > | proc_reg_write+0x60/0xa8 > > > | __vfs_write+0x18/0x40 > > > | vfs_write+0xa4/0x1b8 > > > | ksys_write+0x64/0xf0 > > > | __arm64_sys_write+0x14/0x20 > > > | el0_svc_common.constprop.0+0xb0/0x168 > > > | el0_svc_handler+0x28/0x78 > > > | el0_svc+0x8/0xc > > > | Kernel Offset: disabled > > > | CPU features: 0x0002,24002004 > > > | Memory Limit: none > > > | ---[ end Kernel panic - not syncing: sysrq triggered crash ]--- > > > | Ping 2! > > > | Ping 1! > > > | Ping 1! > > > | Ping 2! > > > > > > The issue can also be triggered on x86 kernels if CONFIG_SMP=n, otherwise > > > local interrupts are disabled in 'smp_send_stop()'. > > > > > > Disable preemption in 'panic()' before re-enabling interrupts. > > > > Is this perhaps the correct solution for what commit c39ea0b9dd24 ("panic: > > avoid the extra noise dmesg") was trying to fix? > > Hmm, maybe, although that looks like it's focussed more on irq handling > than preemption. Exactly, the backtrace mentioned in commit c39ea0b9dd24 ("panic: avoid the extra noise dmesg") is printed by wake_up() called from wake_up_klogd_work_func(). It is irq_work. Therefore disabling preemption would not prevent this. > I've deliberately left the irq part alone, since I think > having magic sysrq work via the keyboard interrupt is desirable from the > panic loop. I agree that we should keep sysrq working. One pity thing is that led_panic_blink() in leds/drivers/trigger/ledtrig-panic.c uses workqueues: + led_panic_blink() + led_trigger_event() + led_set_brightness() + schedule_work() It means that it depends on the scheduler. I guess that it does not work in many panic situations. But this patch will always block it. I agree that it is strange that userspace still works at this stage. But does it cause any real problems? Best Regards, Petr 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=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 79746C4360C for ; Fri, 4 Oct 2019 09:11:50 +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 4826A20867 for ; Fri, 4 Oct 2019 09:11:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="VDa21Dyn" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4826A20867 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.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=n7/Ns1ZhSrfzkcu03R0Mbwyklq9fOV8oHshmbu+ulfw=; b=VDa21DyncDngPQ Xe/1K5Jfk1XvLWgJUjBYTVXCsBeuHlmzg2tatuq/scj5gaBMjejmgdhMsmx46hrMuj5+RRQ9gxHN4 rOlPtTROG2BcAbSBKF+yuvnrUFr+ssUFiLCHJpcFaiHgLAHzWYpyFDzvLNSTeo5DKw97wa00Ta5rb Ra8a4hfvcNlMWZ1hm0cBCWmOpsRKF594XyM7FVOz9Gmss5owCah730GjwYvjR439pYBRNhRq9BVJp MtNy97lGPSrTht3dcldRs9+rEGvgfYUFS9U7B6sTV0RPKveuMnIVJuhYjuasvbYlJySIID8VzDb7b bt4oC6g0wIwTk6Ksq8GA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.2 #3 (Red Hat Linux)) id 1iGJcr-0006nl-Pi; Fri, 04 Oct 2019 09:11:49 +0000 Received: from mx2.suse.de ([195.135.220.15] helo=mx1.suse.de) by bombadil.infradead.org with esmtps (Exim 4.92.2 #3 (Red Hat Linux)) id 1iGJcp-0006n3-0u for linux-arm-kernel@lists.infradead.org; Fri, 04 Oct 2019 09:11:48 +0000 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 9FDC6AEFB; Fri, 4 Oct 2019 09:11:43 +0000 (UTC) Date: Fri, 4 Oct 2019 11:11:42 +0200 From: Petr Mladek To: Will Deacon Subject: Re: [PATCH] panic: Ensure preemption is disabled during panic() Message-ID: <20191004091142.57iylai22aqpu6lu@pathway.suse.cz> References: <20191002123538.22609-1-will@kernel.org> <201910021355.E578D2FFAF@keescook> <20191003205633.w26geqhq67u4ysit@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20191003205633.w26geqhq67u4ysit@willie-the-truck> User-Agent: NeoMutt/20170912 (1.9.0) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20191004_021147_357604_EF23EA5D X-CRM114-Status: GOOD ( 20.17 ) 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: Feng Tang , Kees Cook , Greg Kroah-Hartman , contact@xogium.me, Russell King , Steven Rostedt , linux-kernel@vger.kernel.org, Sergey Senozhatsky , Ingo Molnar , stable@vger.kernel.org, Andrew Morton , 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 Thu 2019-10-03 21:56:34, Will Deacon wrote: > Hi Kees, > > On Wed, Oct 02, 2019 at 01:58:46PM -0700, Kees Cook wrote: > > On Wed, Oct 02, 2019 at 01:35:38PM +0100, Will Deacon wrote: > > > Calling 'panic()' on a kernel with CONFIG_PREEMPT=y can leave the > > > calling CPU in an infinite loop, but with interrupts and preemption > > > enabled. From this state, userspace can continue to be scheduled, > > > despite the system being "dead" as far as the kernel is concerned. This > > > is easily reproducible on arm64 when booting with "nosmp" on the command > > > line; a couple of shell scripts print out a periodic "Ping" message > > > whilst another triggers a crash by writing to /proc/sysrq-trigger: > > > > > > | sysrq: Trigger a crash > > > | Kernel panic - not syncing: sysrq triggered crash > > > | CPU: 0 PID: 1 Comm: init Not tainted 5.2.15 #1 > > > | Hardware name: linux,dummy-virt (DT) > > > | Call trace: > > > | dump_backtrace+0x0/0x148 > > > | show_stack+0x14/0x20 > > > | dump_stack+0xa0/0xc4 > > > | panic+0x140/0x32c > > > | sysrq_handle_reboot+0x0/0x20 > > > | __handle_sysrq+0x124/0x190 > > > | write_sysrq_trigger+0x64/0x88 > > > | proc_reg_write+0x60/0xa8 > > > | __vfs_write+0x18/0x40 > > > | vfs_write+0xa4/0x1b8 > > > | ksys_write+0x64/0xf0 > > > | __arm64_sys_write+0x14/0x20 > > > | el0_svc_common.constprop.0+0xb0/0x168 > > > | el0_svc_handler+0x28/0x78 > > > | el0_svc+0x8/0xc > > > | Kernel Offset: disabled > > > | CPU features: 0x0002,24002004 > > > | Memory Limit: none > > > | ---[ end Kernel panic - not syncing: sysrq triggered crash ]--- > > > | Ping 2! > > > | Ping 1! > > > | Ping 1! > > > | Ping 2! > > > > > > The issue can also be triggered on x86 kernels if CONFIG_SMP=n, otherwise > > > local interrupts are disabled in 'smp_send_stop()'. > > > > > > Disable preemption in 'panic()' before re-enabling interrupts. > > > > Is this perhaps the correct solution for what commit c39ea0b9dd24 ("panic: > > avoid the extra noise dmesg") was trying to fix? > > Hmm, maybe, although that looks like it's focussed more on irq handling > than preemption. Exactly, the backtrace mentioned in commit c39ea0b9dd24 ("panic: avoid the extra noise dmesg") is printed by wake_up() called from wake_up_klogd_work_func(). It is irq_work. Therefore disabling preemption would not prevent this. > I've deliberately left the irq part alone, since I think > having magic sysrq work via the keyboard interrupt is desirable from the > panic loop. I agree that we should keep sysrq working. One pity thing is that led_panic_blink() in leds/drivers/trigger/ledtrig-panic.c uses workqueues: + led_panic_blink() + led_trigger_event() + led_set_brightness() + schedule_work() It means that it depends on the scheduler. I guess that it does not work in many panic situations. But this patch will always block it. I agree that it is strange that userspace still works at this stage. But does it cause any real problems? Best Regards, Petr _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel