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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6B57EC19F2C for ; Fri, 29 Jul 2022 15:47:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238000AbiG2Pr5 (ORCPT ); Fri, 29 Jul 2022 11:47:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50748 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237977AbiG2Prz (ORCPT ); Fri, 29 Jul 2022 11:47:55 -0400 Received: from alexa-out-sd-01.qualcomm.com (alexa-out-sd-01.qualcomm.com [199.106.114.38]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7EC5287F4A for ; Fri, 29 Jul 2022 08:47:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1659109674; x=1690645674; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=57j7+Rr2khf+4no1MtJTPN4zj/VUnqBTlsqYu5nPK68=; b=HopuDYoCX5/U2Wypoqiu8Dk69YtUB3i8uLCrVBmXKu33dzd/597ZaYTb NHfPHYLllS9SyYRuXuPFxFOaF7DaDKv9Um9eWbmUoAM8c06MCenRHcXMj E2L1pcCrpxTLC35b91/htvkg3VqmeoLFTdw90xYb81cGI34BZC/ktyj8E 4=; Received: from unknown (HELO ironmsg05-sd.qualcomm.com) ([10.53.140.145]) by alexa-out-sd-01.qualcomm.com with ESMTP; 29 Jul 2022 08:47:54 -0700 X-QCInternal: smtphost Received: from nasanex01c.na.qualcomm.com ([10.47.97.222]) by ironmsg05-sd.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Jul 2022 08:47:53 -0700 Received: from nalasex01a.na.qualcomm.com (10.47.209.196) by nasanex01c.na.qualcomm.com (10.47.97.222) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.22; Fri, 29 Jul 2022 08:47:53 -0700 Received: from [10.216.48.47] (10.80.80.8) by nalasex01a.na.qualcomm.com (10.47.209.196) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.22; Fri, 29 Jul 2022 08:47:48 -0700 Message-ID: <6b646ff2-b6f6-052e-f3f4-3bf05243f049@quicinc.com> Date: Fri, 29 Jul 2022 21:17:44 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH V2] mm: fix use-after free of page_ext after race with memory-offline Content-Language: en-US To: Michal Hocko CC: , , , , , , , , , , , References: <1658931303-17024-1-git-send-email-quic_charante@quicinc.com> From: Charan Teja Kalla In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nalasex01a.na.qualcomm.com (10.47.209.196) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thanks Michal for the reviews!! On 7/28/2022 8:07 PM, Michal Hocko wrote: >> FAQ's: >> Q) Should page_ext_[get|put]() needs to be used for every page_ext >> access? >> A) NO, the synchronization is really not needed in all the paths of >> accessing page_ext. One case is where extra refcount is taken on a >> page for which memory block, this pages falls into, offline operation is >> being performed. This extra refcount makes the offline operation not to >> succeed hence the freeing of page_ext. Another case is where the page >> is already being freed and we do reset its page_owner. > This is just subtlety and something that can get misunderstood over > time. Moreover there is no documentation explaining the difference. > What is the reason to have these two different APIs in the first place. > RCU read side is almost zero cost. So what is the point? Currently not all the places where page_ext is being used is put under the rcu_lock. I just used rcu lock in the places where it is possible to have the use-after-free of page_ext. You recommend to use rcu lock while using with page_ext in all the places? My only point here is since there may be a non-atomic context exist across page_ext_get/put() and If users are sure that this page's page_ext will not be freed by parallel offline operation, they need not get the rcu lock. I agree that this can be misunderstood over time, let me check if I can use page_ext_get/put in all the places. >> @@ -57,6 +60,11 @@ static inline void page_ext_init(void) >> >> struct page_ext *lookup_page_ext(const struct page *page); >> >> +static inline bool page_ext_invalid(struct page_ext *page_ext) >> +{ >> + return !page_ext || (((unsigned long)page_ext & PAGE_EXT_INVALID) == 1); >> +} >> + > No real reason to expose this into a header file. Nothing but page_ext.c > should know and care about this. Agree. Will move it accordingly. > >> +static inline struct page_ext *page_ext_get(struct page *page) >> +{ >> + struct page_ext *page_ext; >> + >> + rcu_read_lock(); >> + page_ext = lookup_page_ext(page); >> + if (!page_ext) { >> + rcu_read_unlock(); >> + return NULL; >> + } >> + >> + return page_ext; > If you make this an extern you can actually hide lookup_page_ext and > prevent from future bugs where people are using non serialized API > without realizing that. This design looks good. Let me check the feasibility in its implementation. >> diff --git a/mm/page_ext.c b/mm/page_ext.c >> index 3dc715d..404a2eb 100644 >> --- a/mm/page_ext.c >> +++ b/mm/page_ext.c >> @@ -211,15 +211,17 @@ struct page_ext *lookup_page_ext(const struct page *page) >> { >> unsigned long pfn = page_to_pfn(page); >> struct mem_section *section = __pfn_to_section(pfn); >> + struct page_ext *page_ext = READ_ONCE(section->page_ext); >> + > WARN_ON_ONCE(!rcu_read_lock_held()); Again this requires page_ext usage should be under the rcu lock always by the user. > >> static void *__meminit alloc_page_ext(size_t size, int nid) >> @@ -298,9 +300,26 @@ static void __free_page_ext(unsigned long pfn) >> ms = __pfn_to_section(pfn); >> if (!ms || !ms->page_ext) >> return; >> - base = get_entry(ms->page_ext, pfn); >> + >> + base = READ_ONCE(ms->page_ext); >> + if (page_ext_invalid(base)) >> + base = (void *)base - PAGE_EXT_INVALID; > All page_ext accesses should use the same fetched pointer including the > ms->page_ext check. Also page_ext_invalid _must_ be true here otherwise > something bad is going on so I would go with > if (WARN_ON_ONCE(!page_ext_invalid(base))) > return; > base = (void *)base - PAGE_EXT_INVALID; The roll back operation in the online_page_ext(), where we free the allocated page_ext's, will not have the PAGE_EXT_INVALID flag thus WARN() may not work here. no? > Thanks, Charan