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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 74983C433EF for ; Mon, 13 Jun 2022 21:57:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345600AbiFMV5E (ORCPT ); Mon, 13 Jun 2022 17:57:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35346 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231506AbiFMV5A (ORCPT ); Mon, 13 Jun 2022 17:57:00 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 947A020F41 for ; Mon, 13 Jun 2022 14:56:59 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 40B6AB815E6 for ; Mon, 13 Jun 2022 21:56:58 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E86FBC34114; Mon, 13 Jun 2022 21:56:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1655157417; bh=xfBI10NotpHuaoWxC9izbWWM+RGTYWkFGSRITQSg0Wc=; h=Date:To:From:Subject:From; b=QyQ7O8T3jMRDIJToAsrcKPolqQMNTWmNskkq9I3YJWe3vYQ2aIkQMIJm+NWy1hoT4 D/KTNQRazCJEPdEXv/edlN1tTeRMXkGW0FWomD20bQhphqU6CxxIoaqB7fl82o8gTH 9J4HRxLteF5625e0itBj3QRygB2QoVXxVIVRIvEw= Date: Mon, 13 Jun 2022 14:56:56 -0700 To: mm-commits@vger.kernel.org, yi.zhang@huawei.com, viro@zeniv.linux.org.uk, vbabka@suse.cz, surenb@google.com, skhan@linuxfoundation.org, rppt@kernel.org, peterx@redhat.com, namit@vmware.com, mike.kravetz@oracle.com, mgorman@techsingularity.net, ldv@altlinux.org, jack@suse.cz, hughd@google.com, glebfm@altlinux.org, dave.hansen@linux.intel.com, corbet@lwn.net, charante@codeaurora.org, axelrasmussen@google.com, akpm@linux-foundation.org From: Andrew Morton Subject: + userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch added to mm-unstable branch Message-Id: <20220613215656.E86FBC34114@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The patch titled Subject: userfaultfd: update documentation to describe /dev/userfaultfd has been added to the -mm mm-unstable branch. Its filename is userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Axel Rasmussen Subject: userfaultfd: update documentation to describe /dev/userfaultfd Date: Wed, 1 Jun 2022 14:09:49 -0700 Explain the different ways to create a new userfaultfd, and how access control works for each way. Link: https://lkml.kernel.org/r/20220601210951.3916598-5-axelrasmussen@google.com Signed-off-by: Axel Rasmussen Cc: Al Viro Cc: Charan Teja Kalla Cc: Dave Hansen Cc: Dmitry V. Levin Cc: Gleb Fotengauer-Malinovskiy Cc: Hugh Dickins Cc: Jan Kara Cc: Jonathan Corbet Cc: Mel Gorman Cc: Mike Kravetz Cc: Mike Rapoport Cc: Nadav Amit Cc: Peter Xu Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Zhang Yi Signed-off-by: Andrew Morton --- Documentation/admin-guide/mm/userfaultfd.rst | 40 +++++++++++++++-- Documentation/admin-guide/sysctl/vm.rst | 3 + 2 files changed, 40 insertions(+), 3 deletions(-) --- a/Documentation/admin-guide/mm/userfaultfd.rst~userfaultfd-update-documentation-to-describe-dev-userfaultfd +++ a/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 comp 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 +52,38 @@ is a corner case that would currently re 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 by older kernels, is the userfaultfd(2) syscall. +Access to this is controlled in several ways: + +- By default, the userfaultfd will be able to handle kernel page faults. This + can be disabled by passing in UFFD_USER_MODE_ONLY. + +- If vm.unprivileged_userfaultfd is 0, then the caller must *either* have + CAP_SYS_PTRACE, or pass in UFFD_USER_MODE_ONLY. + +- If vm.unprivileged_userfaultfd is 1, then no particular privilege is needed to + use this syscall, even if UFFD_USER_MODE_ONLY is *not* set. + +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; its benefit is in how access to +creating userfaultfds is controlled. + +Access to /dev/userfaultfd is controlled via normal filesystem permissions +(user/group/mode for example), 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). + +Initializing up 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 --- a/Documentation/admin-guide/sysctl/vm.rst~userfaultfd-update-documentation-to-describe-dev-userfaultfd +++ a/Documentation/admin-guide/sysctl/vm.rst @@ -927,6 +927,9 @@ calls without any restrictions. The default value is 0. +An alternative to this sysctl / the userfaultfd(2) syscall is to create +userfaultfds via /dev/userfaultfd. See +Documentation/admin-guide/mm/userfaultfd.rst. user_reserve_kbytes =================== _ Patches currently in -mm which might be from axelrasmussen@google.com are mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch selftests-vm-add-hugetlb_shared-userfaultfd-test-to-run_vmtestssh.patch userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch userfaultfd-selftests-modify-selftest-to-use-dev-userfaultfd.patch userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch userfaultfd-selftests-make-dev-userfaultfd-testing-configurable.patch selftests-vm-add-dev-userfaultfd-test-cases-to-run_vmtestssh.patch