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[209.132.180.67]) by mx.google.com with ESMTP id q2si1106129pfi.165.2019.04.17.19.51.02; Wed, 17 Apr 2019 19:51:20 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387893AbfDRCuH (ORCPT + 99 others); Wed, 17 Apr 2019 22:50:07 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:58304 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1732737AbfDRCuG (ORCPT ); Wed, 17 Apr 2019 22:50:06 -0400 Received: from DGGEMS401-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 9A6E67DD53DE6614E2DF; Thu, 18 Apr 2019 10:50:04 +0800 (CST) Received: from [127.0.0.1] (10.177.19.219) by DGGEMS401-HUB.china.huawei.com (10.3.19.201) with Microsoft SMTP Server id 14.3.408.0; Thu, 18 Apr 2019 10:49:57 +0800 Subject: Re: kernel BUG at kernel/cred.c:434! To: Casey Schaufler , Paul Moore , Oleg Nesterov , References: <20190415134331.GC22204@redhat.com> <20190415150520.GA13257@redhat.com> <20190417145711.GI32622@redhat.com> <20190417162723.GK32622@redhat.com> <0ca3f4cf-5c64-2fc0-1885-9dbcca2f4b47@schaufler-ca.com> CC: "chengjian (D)" , Kees Cook , NeilBrown , Anna Schumaker , "linux-kernel@vger.kernel.org" , Al Viro , "Xiexiuqi (Xie XiuQi)" , Li Bin , "Jason Yan" , Peter Zijlstra , Ingo Molnar , Linux Security Module list , SELinux , Cheng Jian From: Yang Yingliang Message-ID: <5CB7E5D4.2060703@huawei.com> Date: Thu, 18 Apr 2019 10:49:56 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <0ca3f4cf-5c64-2fc0-1885-9dbcca2f4b47@schaufler-ca.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.19.219] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Casey On 2019/4/18 8:24, Casey Schaufler wrote: > On 4/17/2019 4:39 PM, Paul Moore wrote: >> On Wed, Apr 17, 2019 at 12:27 PM Oleg Nesterov wrote: >>> On 04/17, Paul Moore wrote: >>>> On Wed, Apr 17, 2019 at 10:57 AM Oleg Nesterov >>>> wrote: >>>>> On 04/17, Paul Moore wrote: >>>>>> I'm tempted to simply return an error in selinux_setprocattr() if >>>>>> the task's credentials are not the same as its real_cred; >>>>> What about other modules? I have no idea what smack_setprocattr() is, >>>>> but it too does prepare_creds/commit creds. >>>>> >>>>> it seems that the simplest workaround should simply add the >>>>> additional >>>>> cred == real_cred into proc_pid_attr_write(). >>>> Yes, that is simple, but I worry about what other LSMs might want to >>>> do. While I believe failing if the effective creds are not the same >>>> as the real_creds is okay for SELinux (possibly Smack too), I worry >>>> about what other LSMs may want to do. After all, >>>> proc_pid_attr_write() doesn't change the the creds itself, that is >>>> something the specific LSMs do. >>> Yes, but if proc_pid_attr_write() is called with cred != real_cred then >>> something is already wrong? >> True, or at least I would think so. >> >> Looking at the current tree there are three LSMs which implement >> setprocattr hooks: SELinux, Smack, and AppArmor. I know Casey has >> already mentioned that he wasn't able to trigger the problem in Smack, >> but looking at smack_setprocattr() I see the similar commit_creds() >> usage so I would expect the same problem in Smack; what say you Casey? > > I say that my test program runs without ill effect. I call acct() > with "/proc/self/attr/current", which succeeds and enables accounting > just like it is supposed to. I then have the program open > "/proc/self/attr/current" and read it, all of which goes swimmingly. > When Smack frees a cred it usually does not free any memory of its > own, so it is conceivable that I'm just getting lucky. Or, I may not > have sufficient debug enabled. > >> Looking at apparmor_setprocattr(), it appears that it too could end >> up calling commit_creds() via aa_set_current_hat(). >> >> Since it looks like all three LSMs which implement the setprocattr >> hook are vulnerable I'm open to the idea that proc_pid_attr_write() is >> a better choice for the cred != read_cred check, but I would want to >> make sure John and Casey are okay with that. >> >> John? >> >> Casey? > > I'm fine with the change going into proc_pid_attr_write(). The cred != real_cred checking is not enough. Consider this situation, when doing override, cred, real_cred and new_cred are all same: after override_creds() cred == real_cred == new1_cred after prepare_creds() new2_cred after commit_creds() becasue the check is false, so cred == real_cred == new2_cred after revert_creds() cred == new1_cred, real_cred == new2_cred It will cause cred != real_cred finally. Regards, Yang