From patchwork Mon Aug 8 17:56:13 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12938966 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 6B8FCC00140 for ; Mon, 8 Aug 2022 17:57:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230103AbiHHR5O (ORCPT ); Mon, 8 Aug 2022 13:57:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33592 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244144AbiHHR4i (ORCPT ); Mon, 8 Aug 2022 13:56:38 -0400 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A665918341 for ; Mon, 8 Aug 2022 10:56:31 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id j144-20020a25d296000000b0067ba828624fso6133805ybg.16 for ; Mon, 08 Aug 2022 10:56:31 -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=yr7nZVaG6KnPFzU0GkO8LDPXim3Y8RycylsE6e+KYS8=; b=XTaOh2Emx3RzjrAPUqBKn7IkMuMVZE1sNV0UiYrdi4ST+a2W2wVmMRWlnckX47kh8Z 3pwWPZdwO8GQdIoktR8SRLEK3dSAejavjjUTWBsMyR+Lp5qe4E/lj1k5E4pudhXvTlU1 vWnmtqMlJJxhIo/oHFLmarz9DbuZqb7BY/Hq5+m69WpRoZFozOBzbmm16L/QQMNwNYDw /zjQVMRUVK3zklQ5u/E8z49CYBhRyjzy5hwdW98+CYPELiPv45DHhqY4YhFMqJv/CvHY fo3mzTiobWlSpgKtmrAIQxHOldOvv7ws6xTPxVwYC1JWlOe8DXo3TZGLvcyRrvwKcC6H hdPQ== 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=yr7nZVaG6KnPFzU0GkO8LDPXim3Y8RycylsE6e+KYS8=; b=KSTSB6gDRHzZFhk51He/0uKOnIHb8U5S+4Tty7lGHNspNhjSWzLRsn0123GX6a77A1 e5O3V5AlgwCphkJWrGLNf+ev7wEuT+3iH2hbJJHJIPQk787UiXGPdHwomfAl5TLvm3Oc 30i+TsGdVMEwtsw5FxXMeFqt3tuBGPaJv5bXzpIvWdTN08sbRda5gSq0oVM6K+pfnYbJ 9jaYRIg8Q3jKSEs0dLZ70SJAzHaB0tzq0LujYpueDfOVLxezDgaNZp6IdiORqq03aOph PWcQtOT6nrljutwNcd8eKHaYZrxxmJBRlGsGrA8snNLgzAc6qiQMUHv0OlwMUOsqMP5m kTRg== X-Gm-Message-State: ACgBeo12RrjKRLgYxdRajMbvzqRVGULCfwyC+f+vRC+Ax0BUydzM0MHG kobo1lJIpD3rYF40qFo8e+rvDMDsTHtZt2cXqvda X-Google-Smtp-Source: AA6agR5OFChi2bWVcsCQV2S7ZdR7lgDMmRJT1bj1DNJoylej7bIRyUqOMr7NzN7+osLysSkQ7f1ZEMtCfU38FTFoNzeM X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:7a2a:3bb5:f3a0:3bbc]) (user=axelrasmussen job=sendgmr) by 2002:a25:25d8:0:b0:671:80a8:2d73 with SMTP id l207-20020a2525d8000000b0067180a82d73mr16769495ybl.125.1659981390656; Mon, 08 Aug 2022 10:56:30 -0700 (PDT) Date: Mon, 8 Aug 2022 10:56:13 -0700 In-Reply-To: <20220808175614.3885028-1-axelrasmussen@google.com> Message-Id: <20220808175614.3885028-5-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220808175614.3885028-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.1.559.g78731f0fdb-goog Subject: [PATCH v5 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..a76c9dc1865b 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, then 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 and issuing a +USERFAULTFD_IOC_NEW ioctl to /dev/userfaultfd. 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 f74f722ad702..b3e40b42e1b3 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -927,6 +927,9 @@ 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 ===================