From patchwork Wed Aug 17 21:47:27 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12946512 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E7F08C25B08 for ; Wed, 17 Aug 2022 21:48:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242367AbiHQVr6 (ORCPT ); Wed, 17 Aug 2022 17:47:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49078 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242325AbiHQVrx (ORCPT ); Wed, 17 Aug 2022 17:47:53 -0400 Received: from mail-yw1-x114a.google.com (mail-yw1-x114a.google.com [IPv6:2607:f8b0:4864:20::114a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 59AAFAB41F for ; Wed, 17 Aug 2022 14:47:42 -0700 (PDT) Received: by mail-yw1-x114a.google.com with SMTP id 00721157ae682-333b218f2cbso91356927b3.0 for ; Wed, 17 Aug 2022 14:47:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:from:to:cc; bh=jMyZorWE6E22hV3NDdjIMErSuhZAX9bJlMRQ4lfwoQk=; b=kmf5TPe5TDihxUks/W6HE9xnDr7EKu9fX5X/rvsm0Ya68c0awpeJBVGH0nzmUKuYJ8 rE8tjpC3S/ed1CEBFsuxhHdeIjP56/BSB5drqGzrmIJXsqSJBNKymt6+K0z6vPXyWx3i 0tBuuyCs3uyC0x5ddlwxY2lKwN7wUGTezJaC4PMmelE9aNTy46ALdXCVGFD99eosR85J gV9LNz5Sca6lRKdL7jFoNuTCrfmAaL+Lx/NKNp5PuCO/vmUdvXCvKOciNBepvGWehlrZ 8nDB0O42+9TdWUts8kxMGVRvWkzlxt6ohWl6fGZIeamEURvXy40mRJONvGqeYX0aNkb3 QEog== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:x-gm-message-state:from:to:cc; bh=jMyZorWE6E22hV3NDdjIMErSuhZAX9bJlMRQ4lfwoQk=; b=BCJlgOwcXoO0aQ//uaTKOMtQtp/NuQKWpvtV58pdfzJdXkjBFzogHcS1lmKrRUc/I/ 0LhiVSLGphXDZxsTqFRzae6AKAryJO6oC7KbLd16ea24ZM/jFFRF3C4WBtwuVx1WVQfC y0LMueyxXdKHFpQ5R0V+Hm6YXaRr+lfKl4cuz839caoTzHiQfNHl5+cjQ+Dc0j21eShm 7aGyFOfhFmCwFrm33+b+dhPS4eUm0vmwzo8NjNieei9GxpnHQrbNW/Rbu55Y+5FT31WQ s15DkPmmwLo8X2y+atymCwoyLG9yh2K3093JTWD9ndb+3eftvJJykQWCy2Eil9ACAYnf aT0Q== X-Gm-Message-State: ACgBeo1Vh8mCUe30eJ8ctvkGbug0z/KToHOBxgb2ADrDBAOTnKP1OEYr Rm7DRBThdoyDACuNFw29qNSoL2tzSCXs55/8i1fp X-Google-Smtp-Source: AA6agR6aBMm/RqNkrbkKOZWC8FO8iRne3LMJ4YFzZ+fs5wAz7NO45vvRzpcivQOY2FdD/cwWt1QvlitSsbytGa9Pxd0v X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:2f41:f176:4bac:b729]) (user=axelrasmussen job=sendgmr) by 2002:a81:1dd1:0:b0:335:dd05:372c with SMTP id d200-20020a811dd1000000b00335dd05372cmr159548ywd.342.1660772862105; Wed, 17 Aug 2022 14:47:42 -0700 (PDT) Date: Wed, 17 Aug 2022 14:47:27 -0700 In-Reply-To: <20220817214728.489904-1-axelrasmussen@google.com> Message-Id: <20220817214728.489904-5-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220817214728.489904-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.1.595.g718a3a8f04-goog Subject: [PATCH v6 4/5] userfaultfd: update documentation to describe /dev/userfaultfd From: Axel Rasmussen To: Alexander Viro , Andrew Morton , Dave Hansen , "Dmitry V . Levin" , Gleb Fotengauer-Malinovskiy , Hugh Dickins , Jan Kara , Jonathan Corbet , Mel Gorman , Mike Kravetz , Mike Rapoport , Nadav Amit , Peter Xu , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , zhangyi Cc: Axel Rasmussen , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org Precedence: bulk List-ID: Explain the different ways to create a new userfaultfd, and how access control works for each way. Acked-by: Peter Xu Signed-off-by: Axel Rasmussen --- Documentation/admin-guide/mm/userfaultfd.rst | 41 ++++++++++++++++++-- Documentation/admin-guide/sysctl/vm.rst | 3 ++ 2 files changed, 41 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index 6528036093e1..83f31919ebb3 100644 --- a/Documentation/admin-guide/mm/userfaultfd.rst +++ b/Documentation/admin-guide/mm/userfaultfd.rst @@ -17,7 +17,10 @@ of the ``PROT_NONE+SIGSEGV`` trick. Design ====== -Userfaults are delivered and resolved through the ``userfaultfd`` syscall. +Userspace creates a new userfaultfd, initializes it, and registers one or more +regions of virtual memory with it. Then, any page faults which occur within the +region(s) result in a message being delivered to the userfaultfd, notifying +userspace of the fault. The ``userfaultfd`` (aside from registering and unregistering virtual memory ranges) provides two primary functionalities: @@ -34,12 +37,11 @@ The real advantage of userfaults if compared to regular virtual memory management of mremap/mprotect is that the userfaults in all their operations never involve heavyweight structures like vmas (in fact the ``userfaultfd`` runtime load never takes the mmap_lock for writing). - Vmas are not suitable for page- (or hugepage) granular fault tracking when dealing with virtual address spaces that could span Terabytes. Too many vmas would be needed for that. -The ``userfaultfd`` once opened by invoking the syscall, can also be +The ``userfaultfd``, once created, can also be passed using unix domain sockets to a manager process, so the same manager process could handle the userfaults of a multitude of different processes without them being aware about what is going on @@ -50,6 +52,39 @@ is a corner case that would currently return ``-EBUSY``). API === +Creating a userfaultfd +---------------------- + +There are two ways to create a new userfaultfd, each of which provide ways to +restrict access to this functionality (since historically userfaultfds which +handle kernel page faults have been a useful tool for exploiting the kernel). + +The first way, supported since userfaultfd was introduced, is the +userfaultfd(2) syscall. Access to this is controlled in several ways: + +- Any user can always create a userfaultfd which traps userspace page faults + only. Such a userfaultfd can be created using the userfaultfd(2) syscall + with the flag UFFD_USER_MODE_ONLY. + +- In order to also trap kernel page faults for the address space, either the + process needs the CAP_SYS_PTRACE capability, or the system must have + vm.unprivileged_userfaultfd set to 1. By default, vm.unprivileged_userfaultfd + is set to 0. + +The second way, added to the kernel more recently, is by opening +/dev/userfaultfd and issuing a USERFAULTFD_IOC_NEW ioctl to it. This method +yields equivalent userfaultfds to the userfaultfd(2) syscall. + +Unlike userfaultfd(2), access to /dev/userfaultfd is controlled via normal +filesystem permissions (user/group/mode), which gives fine grained access to +userfaultfd specifically, without also granting other unrelated privileges at +the same time (as e.g. granting CAP_SYS_PTRACE would do). Users who have access +to /dev/userfaultfd can always create userfaultfds that trap kernel page faults; +vm.unprivileged_userfaultfd is not considered. + +Initializing a userfaultfd +-------------------------- + When first opened the ``userfaultfd`` must be enabled invoking the ``UFFDIO_API`` ioctl specifying a ``uffdio_api.api`` value set to ``UFFD_API`` (or a later API version) which will specify the ``read/POLLIN`` protocol diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index 9b833e439f09..988f6a4c8084 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -926,6 +926,9 @@ calls without any restrictions. The default value is 0. +Another way to control permissions for userfaultfd is to use +/dev/userfaultfd instead of userfaultfd(2). See +Documentation/admin-guide/mm/userfaultfd.rst. user_reserve_kbytes ===================