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=-11.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 CBD49C433E7 for ; Mon, 12 Oct 2020 10:26:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C2512076D for ; Mon, 12 Oct 2020 10:26:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="B645MlSP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387463AbgJLK0K (ORCPT ); Mon, 12 Oct 2020 06:26:10 -0400 Received: from mx2.suse.de ([195.135.220.15]:38744 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726510AbgJLK0K (ORCPT ); Mon, 12 Oct 2020 06:26:10 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1602498368; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ilXt6NkD7kYkq9BqAajphKCLvAtvM4L7108qoj49GGw=; b=B645MlSP9esJa/Qq1W/IlcqcC4DNU8k3WeD7ZhM/HFe+lcuUjg6FrL1efaU9tEs8vp7SmV y0vWLxlB9hmdQQ3ehDoqBb+Nn4LdH/QvcZxiEhzAfYljTGWSwhdSFPuUnv6zXcTIPBXgeR 3ivv4T/Cu9tAFcTgdE2u/Dd1GkwFvmw= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 62A3EB1E4; Mon, 12 Oct 2020 10:26:08 +0000 (UTC) Subject: Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode To: Peter Zijlstra Cc: xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Andy Lutomirski References: <20201009144225.12019-1-jgross@suse.com> <20201012101330.GR2628@hirez.programming.kicks-ass.net> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: Date: Mon, 12 Oct 2020 12:26:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <20201012101330.GR2628@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12.10.20 12:13, Peter Zijlstra wrote: > On Fri, Oct 09, 2020 at 04:42:25PM +0200, Juergen Gross wrote: >> When running in lazy TLB mode the currently active page tables might >> be the ones of a previous process, e.g. when running a kernel thread. >> >> This can be problematic in case kernel code is being modified via >> text_poke() in a kernel thread, and on another processor exit_mmap() >> is active for the process which was running on the first cpu before >> the kernel thread. >> >> As text_poke() is using a temporary address space and the former >> address space (obtained via cpu_tlbstate.loaded_mm) is restored >> afterwards, there is a race possible in case the cpu on which >> exit_mmap() is running wants to make sure there are no stale >> references to that address space on any cpu active (this e.g. is >> required when running as a Xen PV guest, where this problem has been >> observed and analyzed). >> >> In order to avoid that, drop off TLB lazy mode before switching to the >> temporary address space. > > Oh man, that must've been 'fun' :/ Yeah. > >> Fixes: cefa929c034eb5d ("x86/mm: Introduce temporary mm structs") >> Signed-off-by: Juergen Gross >> --- >> arch/x86/kernel/alternative.c | 9 +++++++++ >> 1 file changed, 9 insertions(+) >> >> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c >> index cdaab30880b9..cd6be6f143e8 100644 >> --- a/arch/x86/kernel/alternative.c >> +++ b/arch/x86/kernel/alternative.c >> @@ -807,6 +807,15 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm) >> temp_mm_state_t temp_state; >> >> lockdep_assert_irqs_disabled(); >> + >> + /* >> + * Make sure not to be in TLB lazy mode, as otherwise we'll end up >> + * with a stale address space WITHOUT being in lazy mode after >> + * restoring the previous mm. >> + */ >> + if (this_cpu_read(cpu_tlbstate.is_lazy)) >> + leave_mm(smp_processor_id()); >> + >> temp_state.mm = this_cpu_read(cpu_tlbstate.loaded_mm); >> switch_mm_irqs_off(NULL, mm, current); > > Would it make sense to write it like: > > this_state.mm = this_cpu_read(cpu_tlbstate.is_lazy) ? > &init_mm : this_cpu_read(cpu_tlbstate.loaded_mm); > > Possibly with that wrapped in a conveniently named helper function. Fine with me, but I don't think it matters that much. For each batch of text_poke() it will be hit only once, and I'm not sure it is really a good idea to use the knowledge that leave_mm() is just a switch to init_mm here. In case it is still the preferred way to do it I can send an update of the patch. Juergen