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=-12.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 A43D8C432BE for ; Mon, 23 Aug 2021 07:52:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B9C86127B for ; Mon, 23 Aug 2021 07:52:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235261AbhHWHwm (ORCPT ); Mon, 23 Aug 2021 03:52:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50064 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235115AbhHWHwj (ORCPT ); Mon, 23 Aug 2021 03:52:39 -0400 Received: from ha0.nfschina.com (unknown [IPv6:2400:dd01:100f:2:d63d:7eff:fe08:eb3f]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 6C45CC061575; Mon, 23 Aug 2021 00:51:57 -0700 (PDT) Received: from localhost (unknown [127.0.0.1]) by ha0.nfschina.com (Postfix) with ESMTP id 0D4A5AE0DBF; Mon, 23 Aug 2021 15:51:40 +0800 (CST) X-Virus-Scanned: amavisd-new at test.com Received: from ha0.nfschina.com ([127.0.0.1]) by localhost (ha0.nfschina.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id zhEQyB8ywpuh; Mon, 23 Aug 2021 15:51:20 +0800 (CST) Received: from [172.30.18.174] (unknown [180.167.10.98]) (Authenticated sender: liqiong@nfschina.com) by ha0.nfschina.com (Postfix) with ESMTPA id 8AF07AE0DA2; Mon, 23 Aug 2021 15:51:19 +0800 (CST) Subject: Re: [PATCH] ima: fix infinite loop within "ima_match_policy" function. From: =?UTF-8?B?5p2O5Yqb55C8?= To: Mimi Zohar , THOBY Simon Cc: "dmitry.kasatkin@gmail.com" , "jmorris@namei.org" , "serge@hallyn.com" , "linux-integrity@vger.kernel.org" , "linux-security-module@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <20210819101529.28001-1-liqiong@nfschina.com> <8d17f252-4a93-f430-3f25-e75556ab01e8@viveris.fr> <1f631c3d-5dce-e477-bfb3-05aa38836442@viveris.fr> <96037695de6125c701889c168550def278adfd4b.camel@linux.ibm.com> Message-ID: <0b0ca83f-9c5b-e4b0-f2c2-b389053479c7@nfschina.com> Date: Mon, 23 Aug 2021 15:51:35 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Simon : Using a temporary ima_rules variable is not working for "ima_policy_next". void *ima_policy_next(struct seq_file *m, void *v, loff_t *pos) { struct ima_rule_entry *entry = v; + struct list_head *ima_rules_tmp = rcu_dereference(ima_rules); rcu_read_lock(); entry = list_entry_rcu(entry->list.next, struct ima_rule_entry, list); rcu_read_unlock(); (*pos)++; - return (&entry->list == ima_rules) ? NULL : entry; + return (&entry->list == ima_rules_tmp) ? NULL : entry; } It seems no way to fix "ima_rules" change within this function, it will alway return a entry if "ima_rules" being changed. Regrads, liqiong 在 2021年08月23日 11:04, 李力琼 写道: > Hi Mimi : > > The situation is a little different,'list_splice_init_rcu' > don't change the list head. If "ima_rules" being changed, > readers may can't reload the new value in time for cpu cache > or compiler optimization. Defining "ima_rules" as a volatile > variable can fix, but It is inefficient. > > Maybe using a temporary ima_rules variable for every > "list_for_each_entry_rcu(entry, ima_rules, list)" loop is > a better solution to fix the "endless loop" bug. > > Regards, > > liqiong > > 在 2021年08月20日 23:48, Mimi Zohar 写道: >> On Fri, 2021-08-20 at 13:23 +0000, THOBY Simon wrote: >>> Hi Liqiong, >>> >>> On 8/20/21 12:15 PM, 李力琼 wrote: >>>> Hi, Simon: >>>> >>>> This solution is better then rwsem, a temp "ima_rules" variable should >>>> can fix. I also have a another idea, with a little trick, default list >>>> can traverse to the new list, so we don't need care about the read side. >>>> >>>> here is the patch: >>>> >>>> @@ -918,8 +918,21 @@ void ima_update_policy(void) >>>> list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu); >>>> >>>> if (ima_rules != policy) { >>>> + struct list_head *prev_rules = ima_rules; >>>> + struct list_head *first = ima_rules->next; >>>> ima_policy_flag = 0; >>>> + >>>> + /* >>>> + * Make the previous list can traverse to new list, >>>> + * that is tricky, or there is a deadly loop whithin >>>> + * "list_for_each_entry_rcu(entry, ima_rules, list)" >>>> + * >>>> + * After update "ima_rules", restore the previous list. >>>> + */ >>> I think this could be rephrased to be a tad clearer, I am not quite sure >>> how I must interpret the first sentence of the comment. >>> >>> >>>> + prev_rules->next = policy->next; >>>> ima_rules = policy; >>>> + syncchronize_rcu(); >>> I'm a bit puzzled as you seem to imply in the mail this patch was tested, >>> but there is no 'syncchronize_rcu' (with two 'c') symbol in the kernel. >>> Was that a copy/paste error? Or maybe you forgot the 'not' in "This >>> patch has been tested"? These errors happen, and I am myself quite an >>> expert in doing them :) >>> >>>> + prev_rules->next = first; >>>> >>>> >>>> The side effect is the "ima_default_rules" will be changed a little while. >>>> But it make sense, the process should be checked again by the new policy. >>>> >>>> This patch has been tested, if will do, I can resubmit this patch.> >>>> How about this ? >>> least >>> >>> Correct me if I'm wrong, here is how I think I understand you patch. >>> We start with a situation like that (step 0): >>> ima_rules --> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0 >>> >>> Then we decide to update the policy for the first time, so >>> 'ima_rules [&ima_default_rules] != policy [&ima_policy_rules]'. >>> We enter the condition. >>> First we copy the current value of ima_rules (&ima_default_rules) >>> to a temporary variable 'prev_rules'. We also create a pointer dubbed >>> 'first' to the entry 1 in the default list (step 1): >>> prev_rules ------------- >>> \/ >>> ima_rules --> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0 >>> /\ >>> first -------------------------------------------------------------- >>> >>> >>> Then we update prev_rules->next to point to policy->next (step 2): >>> List entry 1 <-> List entry 2 <-> ... -> List entry 0 >>> /\ >>> first >>> (notice that list entry 0 no longer points backwards to 'list entry 1', >>> but I don't think there is any reverse iteration in IMA, so it should be >>> safe) >>> >>> prev_rules ------------- >>> \/ >>> ima_rules --> List entry 0 (head node) = ima_default_rules >>> | >>> | >>> ------------------------------------------- >>> \/ >>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0' >>> >>> >>> We then update ima_rules to point to ima_policy_rules (step 3): >>> List entry 1 <-> List entry 2 <-> ... -> List entry 0 >>> /\ >>> first >>> >>> prev_rules ------------- >>> \/ >>> ima_rules List entry 0 (head node) = ima_default_rules >>> | | >>> | | >>> | ------------------------------------------ >>> --------------- | >>> \/ \/ >>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0' >>> synchronize_rcu /\ >>> first -------------------------------------------------------------- >>> >>> Then we run synchronize_rcu() to wait for any RCU reader to exit their loops (step 4). >>> >>> Finally we update prev_rules->next to point back to the ima policy and fix the loop (step 5): >>> >>> List entry 1 <-> List entry 2 <-> ... -> List entry 0 >>> /\ >>> first >>> >>> prev_rules ---> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0 >>> /\ >>> first (now useless) >>> ima_rules >>> | >>> | >>> | >>> --------------- >>> \/ >>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0' >>> >>> The goal is that readers should still be able to loop >>> (forward, as we saw that backward looping is temporarily broken) >>> while in steps 0-4. >>> >>> I'm not completely sure what would happen to a client that started iterating >>> over ima_rules right after step 2. >>> >>> Wouldn't they be able to start looping through the new policy >>> as 'List entry 0 (head node) = ima_default_rules' points to ima_policy_rules? >>> And if they, wouldn't they loop until the write to 'ima_rule' at step 3 (admittedly >>> very shortly thereafter) completed? >>> And would the compiler be allowed to optimize the read to 'ima_rules' in the >>> list_for_each_entry() loop, thereby never reloading the new value for >>> 'ima_rules', and thus looping forever, just what we are trying to avoid? >>> >>> Overall, I'm tempted to say this is perhaps a bit too complex (at least, >>> my head tells me it is, but that may very well be because I'm terrible >>> at concurrency issues). >>> >>> Honestly, in this case I think awaiting input from more experienced >>> kernel devs than I is the best path forward :-) >> I'm far from an expert on RCU locking, but __list_splice_init_rcu() >> provides an example of how to make sure there aren't any readers >> traversing the list, before two lists are spliced together. In our >> case, after there aren't any readers, instead of splicing two lists >> together, it should be safe to point to the new list. >> >> thanks, >> >> Mimi >>