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 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 D7491C433E0 for ; Tue, 5 Jan 2021 06:00:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8BECC229C4 for ; Tue, 5 Jan 2021 06:00:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726658AbhAEGAn (ORCPT ); Tue, 5 Jan 2021 01:00:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34160 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725919AbhAEGAn (ORCPT ); Tue, 5 Jan 2021 01:00:43 -0500 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B17B0C061793; Mon, 4 Jan 2021 22:00:02 -0800 (PST) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kwfNY-006yqt-0a; Tue, 05 Jan 2021 05:59:36 +0000 Date: Tue, 5 Jan 2021 05:59:35 +0000 From: Al Viro To: Stephen Brennan Cc: Alexey Dobriyan , James Morris , "Serge E. Hallyn" , linux-security-module@vger.kernel.org, Paul Moore , Stephen Smalley , Eric Paris , selinux@vger.kernel.org, Casey Schaufler , Eric Biederman , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox Subject: Re: [PATCH v4] proc: Allow pid_revalidate() during LOOKUP_RCU Message-ID: <20210105055935.GT3579531@ZenIV.linux.org.uk> References: <20210104232123.31378-1-stephen.s.brennan@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210104232123.31378-1-stephen.s.brennan@oracle.com> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 04, 2021 at 03:21:22PM -0800, Stephen Brennan wrote: > The pid_revalidate() function drops from RCU into REF lookup mode. When > many threads are resolving paths within /proc in parallel, this can > result in heavy spinlock contention on d_lockref as each thread tries to > grab a reference to the /proc dentry (and drop it shortly thereafter). > > Investigation indicates that it is not necessary to drop RCU in > pid_revalidate(), as no RCU data is modified and the function never > sleeps. So, remove the LOOKUP_RCU check. Umm... I'm rather worried about the side effect you are removing here - you are suddenly exposing a bunch of methods in there to RCU mode. E.g. is proc_pid_permission() safe with MAY_NOT_BLOCK in the mask? generic_permission() call in there is fine, but has_pid_permission() doesn't even see the mask. Is that thing safe in RCU mode? AFAICS, this static int selinux_ptrace_access_check(struct task_struct *child, unsigned int mode) { u32 sid = current_sid(); u32 csid = task_sid(child); if (mode & PTRACE_MODE_READ) return avc_has_perm(&selinux_state, sid, csid, SECCLASS_FILE, FILE__READ, NULL); return avc_has_perm(&selinux_state, sid, csid, SECCLASS_PROCESS, PROCESS__PTRACE, NULL); } is reachable and IIRC avc_has_perm() should *NOT* be called in RCU mode. If nothing else, audit handling needs care... And LSM-related stuff is only a part of possible issues here. It does need a careful code audit - you are taking a bunch of methods into the conditions they'd never been tested in. ->permission(), ->get_link(), ->d_revalidate(), ->d_hash() and ->d_compare() instances for objects that subtree. The last two are not there in case of anything in /proc/, but the first 3 very much are.