From patchwork Fri Aug 19 20:52:00 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12949221 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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DC077C32772 for ; Fri, 19 Aug 2022 20:52:21 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3DEC88E0001; Fri, 19 Aug 2022 16:52:21 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 38EDA8D0003; Fri, 19 Aug 2022 16:52:21 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 27C2D8E0001; Fri, 19 Aug 2022 16:52:21 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 1A6FF8D0003 for ; Fri, 19 Aug 2022 16:52:21 -0400 (EDT) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id E6B11C0341 for ; Fri, 19 Aug 2022 20:52:20 +0000 (UTC) X-FDA: 79817540040.28.F25DADF Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) by imf23.hostedemail.com (Postfix) with ESMTP id 886C1140008 for ; Fri, 19 Aug 2022 20:52:20 +0000 (UTC) Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-337ed9110c2so51034067b3.15 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=w8lwzwgMt+TKuFyk+/r6W9/izWPLKsV0otNZWkOM+UV6La3oCNMuCEl/qPY8POYXv1 8a5k5Li9a4QWe5msMrUZkHS2R2AS8qFR2lc2srZvS9bhY3QxpdJLkAsAtWQepANDGS8H /1X7LHhb9uwoRyjIuzU+8obRLzWnBvdj3ZZuuUhZcDr0tvqi3qKIzUdzgnAo+zAB+ZCh L/psw27NzdDmatYjIgAgoc6h7nEXdxswmWU+RacFJf+rTDz7DaljD4lwEzTMc9r6oQLC tZI53kNHYgedJtRW886S99RxJ95j/ANJs5SboPFE+8yypcFC3Dzn/IBep7zSQgFspKj8 OdEA== X-Gm-Message-State: ACgBeo3H+IZTQeQrNNqD7y0J3Lw3R9fxA7KWVEExioW3FLcHKSVw31oE w1KEF006qGZig8AClHYiC8+4rEVXFJm9uvydD51C 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 ARC-Authentication-Results: i=1; imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=IpWUwfsP; spf=pass (imf23.hostedemail.com: domain of 3A_j_Yg0KCBIsFw39sA4CAAw5y66y3w.u64305CF-442Dsu2.69y@flex--axelrasmussen.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3A_j_Yg0KCBIsFw39sA4CAAw5y66y3w.u64305CF-442Dsu2.69y@flex--axelrasmussen.bounces.google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660942340; a=rsa-sha256; cv=none; b=w3oUvFWjHsHaUwKj9KNtedPKaS4VSKx7PJxToc2uV5TpBsETL5NFkQJNVNJ4emuTg8QyLv 1rR6nhRCzz3oOpROHkJ2dTPQvoCYkBz/aPbqMzuCZ35JaB1gvb67QQN+sdIKwKXhjB1gUq K99Ay9240kZpOO2hmB09ZI7DwKIZ34w= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660942340; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=jMyZorWE6E22hV3NDdjIMErSuhZAX9bJlMRQ4lfwoQk=; b=mhWVlF7Rr8iFSEKFXRijcQM+MjHzLDwqaY5JjTAFH7XHLhuj0NBULmOx7Q1G6NfsWWcWuk drEm63ev3BFC+kLyRkS6ptWOe9nbsafRboXHgMA7h1VVV6ICtxpkN1JgGmcarpSSwI8I/G 9CQo8JR3bEtZdFfVNBTO79Go4VXqg20= X-Rspam-User: Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=IpWUwfsP; spf=pass (imf23.hostedemail.com: domain of 3A_j_Yg0KCBIsFw39sA4CAAw5y66y3w.u64305CF-442Dsu2.69y@flex--axelrasmussen.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3A_j_Yg0KCBIsFw39sA4CAAw5y66y3w.u64305CF-442Dsu2.69y@flex--axelrasmussen.bounces.google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam02 X-Stat-Signature: pb3er98mx78h9ucge6sx83j4qp6rymhi X-Rspamd-Queue-Id: 886C1140008 X-HE-Tag: 1660942340-598758 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org 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 ===================