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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 BC745C15502 for ; Thu, 4 Mar 2021 00:20:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9716F64E60 for ; Thu, 4 Mar 2021 00:20:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1450719AbhCDAUA (ORCPT ); Wed, 3 Mar 2021 19:20:00 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:47154 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1452456AbhCCPvQ (ORCPT ); Wed, 3 Mar 2021 10:51:16 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out02.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1lHTlY-00FcgR-S5; Wed, 03 Mar 2021 08:50:25 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=fess.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1lHTlX-0000vV-Ri; Wed, 03 Mar 2021 08:50:24 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Ilya Lipnitskiy Cc: linux-mm@kvack.org, Linux Kernel Mailing List , linux-fsdevel , Kees Cook , Christoph Hellwig , Linus Torvalds References: Date: Wed, 03 Mar 2021 09:50:23 -0600 In-Reply-To: (Ilya Lipnitskiy's message of "Tue, 2 Mar 2021 23:01:16 -0800") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1lHTlX-0000vV-Ri;;;mid=;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX19zA/lsFUb7Jgarg+h7HqfyzQCN+3IxXT8= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: exec error: BUG: Bad rss-counter X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Ilya Lipnitskiy writes: > On Tue, Mar 2, 2021 at 11:37 AM Eric W. Biederman wrote: >> >> Ilya Lipnitskiy writes: >> >> > On Mon, Mar 1, 2021 at 12:43 PM Eric W. Biederman wrote: >> >> >> >> Ilya Lipnitskiy writes: >> >> >> >> > Eric, All, >> >> > >> >> > The following error appears when running Linux 5.10.18 on an embedded >> >> > MIPS mt7621 target: >> >> > [ 0.301219] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:1 >> >> > >> >> > Being a very generic error, I started digging and added a stack dump >> >> > before the BUG: >> >> > Call Trace: >> >> > [<80008094>] show_stack+0x30/0x100 >> >> > [<8033b238>] dump_stack+0xac/0xe8 >> >> > [<800285e8>] __mmdrop+0x98/0x1d0 >> >> > [<801a6de8>] free_bprm+0x44/0x118 >> >> > [<801a86a8>] kernel_execve+0x160/0x1d8 >> >> > [<800420f4>] call_usermodehelper_exec_async+0x114/0x194 >> >> > [<80003198>] ret_from_kernel_thread+0x14/0x1c >> >> > >> >> > So that's how I got to looking at fs/exec.c and noticed quite a few >> >> > changes last year. Turns out this message only occurs once very early >> >> > at boot during the very first call to kernel_execve. current->mm is >> >> > NULL at this stage, so acct_arg_size() is effectively a no-op. >> >> >> >> If you believe this is a new error you could bisect the kernel >> >> to see which change introduced the behavior you are seeing. >> >> >> >> > More digging, and I traced the RSS counter increment to: >> >> > [<8015adb4>] add_mm_counter_fast+0xb4/0xc0 >> >> > [<80160d58>] handle_mm_fault+0x6e4/0xea0 >> >> > [<80158aa4>] __get_user_pages.part.78+0x190/0x37c >> >> > [<8015992c>] __get_user_pages_remote+0x128/0x360 >> >> > [<801a6d9c>] get_arg_page+0x34/0xa0 >> >> > [<801a7394>] copy_string_kernel+0x194/0x2a4 >> >> > [<801a880c>] kernel_execve+0x11c/0x298 >> >> > [<800420f4>] call_usermodehelper_exec_async+0x114/0x194 >> >> > [<80003198>] ret_from_kernel_thread+0x14/0x1c >> >> > >> >> > In fact, I also checked vma_pages(bprm->vma) and lo and behold it is set to 1. >> >> > >> >> > How is fs/exec.c supposed to handle implied RSS increments that happen >> >> > due to page faults when discarding the bprm structure? In this case, >> >> > the bug-generating kernel_execve call never succeeded, it returned -2, >> >> > but I didn't trace exactly what failed. >> >> >> >> Unless I am mistaken any left over pages should be purged by exit_mmap >> >> which is called by mmput before mmput calls mmdrop. >> > Good to know. Some more digging and I can say that we hit this error >> > when trying to unmap PFN 0 (is_zero_pfn(pfn) returns TRUE, >> > vm_normal_page returns NULL, zap_pte_range does not decrement >> > MM_ANONPAGES RSS counter). Is my understanding correct that PFN 0 is >> > usable, but special? Or am I totally off the mark here? >> >> It would be good to know if that is the page that get_user_pages_remote >> returned to copy_string_kernel. The zero page that is always zero, >> should never be returned when a writable mapping is desired. > > Indeed, pfn 0 is returned from get_arg_page: (page is 0x809cf000, > page_to_pfn(page) is 0) and it is the same page that is being freed and not > refcounted in mmput/zap_pte_range. Confirmed with good old printk. Also, > ZERO_PAGE(0)==0x809fc000 -> PFN 5120. > > I think I have found the problem though, after much digging and thanks to all > the information provided. init_zero_pfn() gets called too late (after > the call to > is_zero_pfn(0) from mmput returns true), until then zero_pfn == 0, and after, > zero_pfn == 5120. Boom. > > So PFN 0 is special, but only for a little bit, enough for something > on my system > to call kernel_execve :) > > Question: is my system not supposed to be calling kernel_execve this > early or does > init_zero_pfn() need to happen earlier? init_zero_pfn is currently a > core_initcall. Looking quickly it seems that init_zero_pfn() is in mm/memory.c and is common for both mips and x86. Further it appears init_zero_pfn() has been that was since 2009 a13ea5b75964 ("mm: reinstate ZERO_PAGE"). Given the testing that x86 gets and that nothing like this has been reported it looks like whatever driver is triggering the kernel_execve is doing something wrong. Because honestly. If the zero page isn't working there is not a chance that anything in userspace is working so it is clearly much too early. I suspect there is some driver that is initialized very early that is doing something that looks innocuous (like triggering a hotplug event) and that happens to cause a call_usermode_helper which then calls kernel_execve. Eric