From nobody Wed Apr 8 14:09:19 2026 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 C3AB5C32771 for ; Fri, 19 Aug 2022 20:53:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352054AbiHSUxw (ORCPT ); Fri, 19 Aug 2022 16:53:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51390 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1352041AbiHSUwf (ORCPT ); Fri, 19 Aug 2022 16:52:35 -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 8A78110E96C for ; Fri, 19 Aug 2022 13:52:20 -0700 (PDT) Received: by mail-yw1-x114a.google.com with SMTP id 00721157ae682-33580e26058so94454327b3.4 for ; Fri, 19 Aug 2022 13:52:20 -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=IpWUwfsPRKJE7OaJsJdB2L08XMnLW2ICg+R0QoqCfLw/E4APvPDrTfhcUirY+CZND6 evb3tGyOoymzo6FCUW8oF7Syaupj6TQjDhLih1PVQf/Rio67h8m/TNR8mc+mNIFCccFO XDR3/ttkxX+6+uyZ0L00oWOJw7Abd3WlwNN0nzKmbpMcBzrOLmiIY2XAchs5p9aaKPyV 19dMLz80LBUqjO9wA3XLdqK8TlQm9heL8kJXn7tdZEh0h1An+wd9Oxwp+/xzzATJG9Wb jFJZoo5Q77AwoC/iF70h0EENSO63wwRgUEvWmas1zzZOsbJjVnw2hhoS71ceZe4QoIX8 uB7Q== 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=yYgO9sopSUkd47qv3q3ehuqTM6cUK7dX5BumVKDoehxMmCdL0wnwWhO/chmBiamb/M E10pRYmgnbnHY/WzMytvI/vsnTDPzo75cTybTe4EisHn7hiewyaksOLnuNimOCDlIMDa 95d2NG5VfOuujFg0AYsx7wkb/DO8UWNXPtV7BgxlHavKP3ur+YNl7MhjYWUHZMAfeQrC CjPgJh6LkIbo5LnVoVMzo6bKGoVtcXDcirz+fA006FU2T0mnVRrw6kl7YFOu6yVLFrUl 97mvjQJt7lOb9BXtTMTvAoqkXq+0Xoz6X9GTvUdGJZKSyJaJ7im3Y+XqrWUE3/5HRhnS DVEw== X-Gm-Message-State: ACgBeo3uUj3o5NMcHw+RDXkVerG0idlGxjaVw6l0L1iY6MIdiu95bY1w 4gzZiiMFnnf/mFYWthcyiBpjjAFVFtnUQWk9oZA8 X-Google-Smtp-Source: AA6agR5HOAyxwvEDm7V3pQX2zZvc4IogiVRAS3GDNSIX/0PRNm7ftLReSWlEyTi49ZVZs+XnFcamtJitzIArA/qTOKgk X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:baf:4c5:18b:2c4b]) (user=axelrasmussen job=sendgmr) by 2002:a0d:eace:0:b0:332:1c2a:6d7c with SMTP id t197-20020a0deace000000b003321c2a6d7cmr9325489ywe.481.1660942339850; Fri, 19 Aug 2022 13:52:19 -0700 (PDT) Date: Fri, 19 Aug 2022 13:52:00 -0700 In-Reply-To: <20220819205201.658693-1-axelrasmussen@google.com> Message-Id: <20220819205201.658693-5-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220819205201.658693-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.1.595.g718a3a8f04-goog Subject: [PATCH v7 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: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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/a= dmin-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 =3D=3D=3D=3D=3D=3D =20 -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 withi= n the +region(s) result in a message being delivered to the userfaultfd, notifying +userspace of the fault. =20 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. =20 -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 =3D=3D=3D =20 +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 whi= ch +handle kernel page faults have been a useful tool for exploiting the kerne= l). + +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 faul= ts + 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 t= he + process needs the CAP_SYS_PTRACE capability, or the system must have + vm.unprivileged_userfaultfd set to 1. By default, vm.unprivileged_userfa= ultfd + 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 a= ccess +to /dev/userfaultfd can always create userfaultfds that trap kernel page f= aults; +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_A= PI`` (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. =20 The default value is 0. =20 +Another way to control permissions for userfaultfd is to use +/dev/userfaultfd instead of userfaultfd(2). See +Documentation/admin-guide/mm/userfaultfd.rst. =20 user_reserve_kbytes =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --=20 2.37.1.595.g718a3a8f04-goog