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 1FC1EC43334 for ; Tue, 19 Jul 2022 22:48:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240391AbiGSWsN (ORCPT ); Tue, 19 Jul 2022 18:48:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46846 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236237AbiGSWsG (ORCPT ); Tue, 19 Jul 2022 18:48:06 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 300274E62F for ; Tue, 19 Jul 2022 15:48:05 -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 1A211B81DA2 for ; Tue, 19 Jul 2022 22:48:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 94040C341CA; Tue, 19 Jul 2022 22:48:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1658270881; bh=j5om20Gj5Tp587v+VYVMc3moOJ9Lu9efQ239YMl10fE=; h=Date:To:From:Subject:From; b=CMxcvzK99RisFg3VIEGjmneKNKUjq5fL9lz+50V3ZEm7WrjV2ez/MELGGCB+q9T4F cI8H0Na9lFyfGU+W0uCNExChkQDDslJJTfU2o53jrmaz7LFizriWIV8cHqbM74tncZ tBem2I0M9SUc2T3eFe1Jqi0GI8x570oqapMqsynM= Date: Tue, 19 Jul 2022 15:48:00 -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: [to-be-updated] userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch removed from -mm tree Message-Id: <20220719224801.94040C341CA@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The quilt patch titled Subject: userfaultfd: update documentation to describe /dev/userfaultfd has been removed from the -mm tree. Its filename was userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch This patch was dropped because an updated version will be merged ------------------------------------------------------ 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 @@ -926,6 +926,9 @@ 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 userfaultfd-selftests-make-dev-userfaultfd-testing-configurable.patch selftests-vm-add-dev-userfaultfd-test-cases-to-run_vmtestssh.patch