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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 7DD78C433E0 for ; Fri, 15 Jan 2021 15:28:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 353A22388B for ; Fri, 15 Jan 2021 15:28:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729243AbhAOP2l (ORCPT ); Fri, 15 Jan 2021 10:28:41 -0500 Received: from mail.skyhub.de ([5.9.137.197]:40588 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729116AbhAOP2k (ORCPT ); Fri, 15 Jan 2021 10:28:40 -0500 Received: from zn.tnic (p200300ec2f0acf00d1d88af93a38b68f.dip0.t-ipconnect.de [IPv6:2003:ec:2f0a:cf00:d1d8:8af9:3a38:b68f]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id 0A1561EC0253; Fri, 15 Jan 2021 16:27:59 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1610724479; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=/UHXUR5eCzgC6qnSSG+fMjiOWrrQoVMo0QLlYLKvhZE=; b=d6lM4sf9U/uQHSIdYXkxEV5PlzOzSSbhpO6qj6aUvv0deQ9rCuXdbjidHuIz2R4FjOaC4t IA7RW4YgzFjBVitmCxoIoWJIbNznTeAYt1vgf6epN1VF0WIXJ/XC6hJLw27vLCRUgdLLlt E1+aB2+4ySOWgIfTVgI/hR5fybk1tp0= Date: Fri, 15 Jan 2021 16:27:54 +0100 From: Borislav Petkov To: Tony Luck Cc: x86@kernel.org, Andrew Morton , Peter Zijlstra , Darren Hart , Andy Lutomirski , linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery Message-ID: <20210115152754.GC9138@zn.tnic> References: <20210111214452.1826-1-tony.luck@intel.com> <20210115003817.23657-1-tony.luck@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210115003817.23657-1-tony.luck@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-edac@vger.kernel.org On Thu, Jan 14, 2021 at 04:38:17PM -0800, Tony Luck wrote: > Recovery action when get_user() triggers a machine check uses the fixup > path to make get_user() return -EFAULT. Also queue_task_work() sets up > so that kill_me_maybe() will be called on return to user mode to send a > SIGBUS to the current process. > > But there are places in the kernel where the code assumes that this > EFAULT return was simply because of a page fault. The code takes some > action to fix that, and then retries the access. This results in a second > machine check. > > While processing this second machine check queue_task_work() is called > again. But since this uses the same callback_head structure that > was used in the first call, the net result is an entry on the > current->task_works list that points to itself. When task_work_run() > is called it loops forever in this code: > > do { > next = work->next; > work->func(work); > work = next; > cond_resched(); > } while (work); > > Add a "mce_busy" counter so that task_work_add() is only called once > per faulty page in this task. Yeah, that sentence can be removed now too. > Do not allow too many repeated machine checks, or machine checks to > a different page from the first. > > Signed-off-by: Tony Luck > --- > > V3: Thanks to extensive commentary from Andy & Boris > > Throws out the changes to get_user() and subsequent changes to core > code. Everything is now handled in the machine check code. Downside is > that we can (and do) take multiple machine checks from a single poisoned > page before generic kernel code finally gets the message that a page is > really and truly gone (but all the failed get_user() calls still return > the legacy -EFAULT code, so none of that code will ever mistakenly use > a value from a bad page). But even on an old machine that does broadcast > interrupts for each machine check things survive multiple cycles of my > test injection into a futex operation. Nice. > > I picked "10" as the magic upper limit for how many times the machine > check code will allow a fault from the same page before deciding to > panic. We can bike shed that value if you like. > > arch/x86/kernel/cpu/mce/core.c | 27 ++++++++++++++++++++------- > include/linux/sched.h | 1 + > 2 files changed, 21 insertions(+), 7 deletions(-) > > diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c > index 13d3f1cbda17..25daf6517dc9 100644 > --- a/arch/x86/kernel/cpu/mce/core.c > +++ b/arch/x86/kernel/cpu/mce/core.c > @@ -1246,6 +1246,7 @@ static void kill_me_maybe(struct callback_head *cb) > struct task_struct *p = container_of(cb, struct task_struct, mce_kill_me); > int flags = MF_ACTION_REQUIRED; > > + p->mce_count = 0; > pr_err("Uncorrected hardware memory error in user-access at %llx", p->mce_addr); > > if (!p->mce_ripv) > @@ -1266,12 +1267,24 @@ static void kill_me_maybe(struct callback_head *cb) > } > } > > -static void queue_task_work(struct mce *m, int kill_current_task) > +static void queue_task_work(struct mce *m, char *msg, int kill_current_task) So this function gets called in the user mode MCE case too: if ((m.cs & 3) == 3) { queue_task_work(&m, msg, kill_current_task); } Do we want to panic for multiple MCEs to different addresses in user mode? I don't think so - that should go down the memory failure page offlining path... > - current->mce_addr = m->addr; > - current->mce_kflags = m->kflags; > - current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV); > - current->mce_whole_page = whole_page(m); > + if (current->mce_count++ == 0) { > + current->mce_addr = m->addr; > + current->mce_kflags = m->kflags; > + current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV); > + current->mce_whole_page = whole_page(m); > + } > + /* Magic number should be large enough */ > + if (current->mce_count > 10) > + mce_panic("Too many machine checks while accessing user data", m, msg); > + > + if (current->mce_count > 1 || (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT)) > + mce_panic("Machine checks to different user pages", m, msg); Will this second part of the test expression, after the "||" ever hit? You do above in the first branch: if (current->mce_count++ == 0) { ... current->mce_addr = m->addr; and ->mce_count becomes 1. In that case that (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT) gets tested but that won't ever be true because ->mce_addr = ->addr above. And then, for other values of mce_count, mce_count > 1 will hit. In any case, what are you trying to catch with this? Two get_user() to different pages both catching MCEs? > + > + /* Do not call task_work_add() more than once */ > + if (current->mce_count > 1) > + return; That won't happen either, AFAICT. It'll panic above. Regardless, I like how this is all confined to the MCE code and there's no need to touch stuff outside... Thx. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette