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,URIBL_BLOCKED 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 9148FC433E0 for ; Fri, 26 Feb 2021 11:14:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4080B64EF0 for ; Fri, 26 Feb 2021 11:14:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231151AbhBZLNq (ORCPT ); Fri, 26 Feb 2021 06:13:46 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:59183 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230153AbhBZLNg (ORCPT ); Fri, 26 Feb 2021 06:13:36 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614337929; 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=j6n53j9KxHkRJ3cFFI4fNEUccn3z7+2i49tGG9CMSsg=; b=AA2+3HNABQOZNjfwkCtr5xj41H1pQnh1ggrHl3YefWKpzTpcRo8HPase+d/bfc/AuHXuPi qJgNfTbzvuzB1TnYCRpkyaJ1o2Og/kgTScWrSrQ4K0Nf+vBz5Fixk8xOfruVOhfySPiaWH D8Tjl42wvu8np4UyQvVbr5biHWHxUS0= Received: from mail-yb1-f197.google.com (mail-yb1-f197.google.com [209.85.219.197]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-246-MXqxFMPLPt6pfBy5KAQ6YQ-1; Fri, 26 Feb 2021 06:12:07 -0500 X-MC-Unique: MXqxFMPLPt6pfBy5KAQ6YQ-1 Received: by mail-yb1-f197.google.com with SMTP id a63so9733837yba.2 for ; Fri, 26 Feb 2021 03:12:07 -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=j6n53j9KxHkRJ3cFFI4fNEUccn3z7+2i49tGG9CMSsg=; b=jmFW6rCvwoCH48FeYLSfQ+EPNC6kjY1epLBMzd/hgrGVHZPwUt0GYKqDO9clMmYI0D 66B7AsJ7Gp8tqcrX832Yaad/ESQnpibUuIAqQ90OnGPArr10mdS3g4HU0FUsF1JQwFe9 9Lpn4X1LFd+MLhWb0Je814S79ipnOIT8CRNTMaJ+jzh30Klzc/rGtgz+RecYHY/sDwpQ aFHSHGl85EwWEDefipWuVQjsYlT8AACvIK/Jde5k7SDAg1ewqfS2uWTMU9MInsq362zT FQuoof5chz7Utkgbb5FGHGgeDYQ3rzeQxB9UHG8LthVQeVPxMRuXpr7ogcsaJO+00Oi/ xjRw== X-Gm-Message-State: AOAM531r8XIjvgf9/mrByUlebVvoQikATX21Ycmayscyo7hMfX1AaqvS 94BAV1Y9xUi5olwydvy3/W/pRnA+1s2RZgKUWQatmrAbKUeS3EEQvn+KWxPKshjBk5JPenq10Js m3gMWpWC+bnN46ZJ4za8jkKlYocwuxln4qVubYQNZ X-Received: by 2002:a25:ad67:: with SMTP id l39mr3741084ybe.172.1614337926806; Fri, 26 Feb 2021 03:12:06 -0800 (PST) X-Google-Smtp-Source: ABdhPJwfdmyB2V3wBE6ftPc+OWfFKzel7ICJXiZyI+YPfu/4JcOrmhdfSffcL8fWvhRbb6NfSZyvDzKX/HibndwRGC8= X-Received: by 2002:a25:ad67:: with SMTP id l39mr3741066ybe.172.1614337926612; Fri, 26 Feb 2021 03:12:06 -0800 (PST) MIME-Version: 1.0 References: <20210223214346.GB6000@sequoia> <20210223215054.GC6000@sequoia> <20210223223652.GD6000@sequoia> In-Reply-To: From: Ondrej Mosnacek Date: Fri, 26 Feb 2021 12:11:56 +0100 Message-ID: Subject: Re: [BUG] Race between policy reload sidtab conversion and live conversion To: Paul Moore Cc: Tyler Hicks , Stephen Smalley , SElinux list , Linux kernel mailing list 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 2:07 AM 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? ;) Sadly, yes :) Consider this scenario (assuming we fix the locking at sidtab level): If it happens that a new SID (x) is added via the new sidtab and then another one (y) via the old sidtab, to avoid clash of SIDs, we would need to leave a "hole" in the old sidtab for SID x. And this will cause trouble if the thread that has just added SID y, then tries to translate the context string corresponding to SID x (without re-taking the RCU read lock and refreshing the policy pointer). Even if we handle skipping the "holes" in the old sidtab safely, the translation would then end up adding a duplicate SID entry for the context already represented by SID x - which is not a state we want to end up in. This is why I said that to fully fix this, we'd need to have a both-ways live conversion in place. (And that already starts to feel like too much hacking for something that should probably go to stable@...) -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.