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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 2946AC43381 for ; Fri, 26 Feb 2021 11:21:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CE69B64E6C for ; Fri, 26 Feb 2021 11:21:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230292AbhBZLVU (ORCPT ); Fri, 26 Feb 2021 06:21:20 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:28648 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230139AbhBZLVP (ORCPT ); Fri, 26 Feb 2021 06:21:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614338388; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=21WHWcxyW0R0i+nu89iK+hY7xxF9KTjB2gFpICOTc2I=; b=P1TiQBc/KH5BmtGi8aPb83HW5vq7BqNpmd40OlipK/LeoFSToRVXe1fC1rBnDW6o7gBM2U AHErgJMKrPiKkmQNkgsvVk9IMu0J0KB8ztiSrY0MqzLh+W/9DNZKochGCWmPPQUroE0E/R CYCpTaci76NxvKA9onStIuTMH4VULQI= Received: from mail-yb1-f199.google.com (mail-yb1-f199.google.com [209.85.219.199]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-353-nmOt5DJEOy-v8JCRDGcwdQ-1; Fri, 26 Feb 2021 06:19:46 -0500 X-MC-Unique: nmOt5DJEOy-v8JCRDGcwdQ-1 Received: by mail-yb1-f199.google.com with SMTP id o9so9595913yba.18 for ; Fri, 26 Feb 2021 03:19:46 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=21WHWcxyW0R0i+nu89iK+hY7xxF9KTjB2gFpICOTc2I=; b=GSSB2Jxo1n4hTYg0VilhEizyZDEkVLSUdQwyqXH3zGdLRA8AimqMAfGGfCSGNjqlWu 2tUhDWQV3psVL1G0MQiV7IRVETZih4lpyotMFpPFVfJRaGK9cBDeY7n/2HLcu25ARqFy V0o1e+y/NbArk9/vUSLzUiHfJUdmjkmOP4V0klujcq9e+viT3X3WIV/vYuOLQ2Srf3IX ll2iyfI2UEpzbo1TvS/knzNq+t28I/0jh72xTtznEhb6EKkRln2cUxiDbJTs71zFDhpu +vLVHkW2VsiEHCeOHtof1JBnfvxnSVQQlR3p2UdUc0J77lhyyTgPkNd84kXUvUp6Ahiy Zbvg== X-Gm-Message-State: AOAM530hIQwiQ8LNsu3SEZg4D36eWsCSlKtvI3bNfFn3Ub4HvA75zidB Zq2poEkKfsNzlZywaMlJafVhnAPVL6whRZnVgCEvJkMQRepEO3aXXXyQbMlyd8hJUejhOOG8Afs T+Tw+r28HgVpoqQG5S/7Ls6UVhzgW+6ritC5imBBy X-Received: by 2002:a25:d104:: with SMTP id i4mr3775493ybg.227.1614338385618; Fri, 26 Feb 2021 03:19:45 -0800 (PST) X-Google-Smtp-Source: ABdhPJwdQiHBNItfITac2JMT1HbsAti2DvmAEICafs5OqhUYvrC/r11A5AD8rclfrmyW0zhJGa3ZvorxhEGSxhnFBN4= X-Received: by 2002:a25:d104:: with SMTP id i4mr3775471ybg.227.1614338385419; Fri, 26 Feb 2021 03:19:45 -0800 (PST) MIME-Version: 1.0 References: <20210223214346.GB6000@sequoia> <20210223215054.GC6000@sequoia> <20210223223652.GD6000@sequoia> <20210226040542.1137-1-hdanton@sina.com> In-Reply-To: <20210226040542.1137-1-hdanton@sina.com> From: Ondrej Mosnacek Date: Fri, 26 Feb 2021 12:19:35 +0100 Message-ID: Subject: Re: [BUG] Race between policy reload sidtab conversion and live conversion To: Hillf Danton Cc: Paul Moore , Tyler Hicks , Stephen Smalley , SElinux list , LKML Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 26, 2021 at 5:08 AM Hillf Danton wrote: > On Thu, 25 Feb 2021 20:06:45 -0500 Paul Moore wrote: > > On Wed, Feb 24, 2021 at 4:35 AM Ondrej Mosnacek wrote: > > > After the switch to RCU, we now have: > > > 1. Start live conversion of new entries. > > > 2. Convert existing entries. > > > 3. RCU-assign the new policy pointer to selinux_state. > > > [!!! Now actually both old and new sidtab may be referenced by > > > readers, since there is no synchronization barrier previously provided > > > by the write lock.] > > > 4. Wait for synchronize_rcu() to return. > > > 5. Now only the new sidtab is visible to readers, so the old one can > > > be destroyed. > > > > > > So the race can happen between 3. and 5., if one thread already sees > > > the new sidtab and adds a new entry there, and a second thread still > > > has the reference to the old sidtab and also tires to add a new entry; > > > live-converting to the new sidtab, which it doesn't expect to change > > > by itself. Unfortunately I failed to realize this when reviewing the > > > patch :/ > > > > It is possible I'm not fully understanding the problem and/or missing > > an important detail - it is rather tricky code, and RCU can be very > > hard to reason at times - but I think we may be able to solve this > > with some lock fixes inside sidtab_context_to_sid(). Let me try to > > explain to see if we are on the same page here ... > > > > The problem is when we have two (or more) threads trying to > > add/convert the same context into a sid; the task with new_sidtab is > > looking to add a new sidtab entry, while the task with old_sidtab is > > looking to convert an entry in old_sidtab into a new entry in > > new_sidtab. Boom. > > > > Looking at the code in sidtab_context_to_sid(), when we have two > > sidtabs that are currently active (old_sidtab->convert pointer is > > valid) and a task with old_sidtab attempts to add a new entry to both > > sidtabs it first adds it to the old sidtab then it also adds it to the > > new sidtab. I believe the problem is that in this case while the task > > grabs the old_sidtab->lock, it never grabs the new_sidtab->lock which > > allows it to race with tasks that already see only new_sidtab. I > > think adding code to sidtab_context_to_sid() which grabs the > > new_sidtab->lock when adding entries to the new_sidtab *should* solve > > the problem. > > > > Did I miss something important? ;) > > If the convert pointer can be derefered without lock, we can opt to > convert context after building sidtab with the risk of AB BA deadlock > cut. Below is the minimum change I can think of along your direction. We could fix this a bit more easily by just having a shared spinlock for both (well, *all*) sidtabs. Yes, we'd need to have it all the way up in selinux_state and pass it through to sidtab_init(), but IMHO that's less bad than trying to get it right with two locks. -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.