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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 Content-Type: text/plain; charset="UTF-8" ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660772862; 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=zN+iJDp/EyekxbKCkOjz+pSd3RRxKo4KfBK7SritKsKI+8oW4ev1vU89D5wgbOqgEWzMjA HvHNWtNHwR6sj76TV+oSa+KU3T/0QmFk+rcXntr0twDQxCulzK67je3iA5amWONdYVIOr9 KVciy0N9+JydlhpqFFSNcd4PCDPng80= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=kmf5TPe5; spf=pass (imf13.hostedemail.com: domain of 3_mH9Yg0KCNU1O5CI1JDLJJ5E7FF7C5.3FDC9ELO-DDBM13B.FI7@flex--axelrasmussen.bounces.google.com designates 209.85.128.201 as permitted sender) smtp.mailfrom=3_mH9Yg0KCNU1O5CI1JDLJJ5E7FF7C5.3FDC9ELO-DDBM13B.FI7@flex--axelrasmussen.bounces.google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660772862; a=rsa-sha256; cv=none; b=cj/cU1sC015SnljikuiRMNz+O9IQB0d/9SsEcEfPCaMTsMBtQqW4Wb/7NK6uVX5KKNQbwb BXWa4plVc+41VaCzfWM8811bdOX1y3KIW6tKzOSbbm5RWiObw8wMuzzudgjRrO1Jc5KzgV KCPQC5WvU9WCFXrjxFmB9F26QqZdL74= Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=kmf5TPe5; spf=pass (imf13.hostedemail.com: domain of 3_mH9Yg0KCNU1O5CI1JDLJJ5E7FF7C5.3FDC9ELO-DDBM13B.FI7@flex--axelrasmussen.bounces.google.com designates 209.85.128.201 as permitted sender) smtp.mailfrom=3_mH9Yg0KCNU1O5CI1JDLJJ5E7FF7C5.3FDC9ELO-DDBM13B.FI7@flex--axelrasmussen.bounces.google.com; dmarc=pass (policy=reject) header.from=google.com X-Stat-Signature: 5i7hpfpoadfa4pox9t8wraakezbyqkqc X-Rspamd-Queue-Id: 7C17820088 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1660772862-575852 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 =================== -- 2.37.1.595.g718a3a8f04-goog