+ userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.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

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------------------------------------------------------
From: Axel Rasmussen <axelrasmussen@google.com>
Subject: userfaultfd: update documentation to describe /dev/userfaultfd
Date: Mon, 8 Aug 2022 10:56:13 -0700

Explain the different ways to create a new userfaultfd, and how access
control works for each way.

Link: https://lkml.kernel.org/r/20220808175614.3885028-5-axelrasmussen@google.com
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dmitry V. Levin <ldv@altlinux.org>
Cc: Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/admin-guide/mm/userfaultfd.rst |   41 +++++++++++++++--
 Documentation/admin-guide/sysctl/vm.rst      |    3 +
 2 files changed, 41 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 +52,39 @@ 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 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
--- 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.
 
+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
 ===================
_

Patches currently in -mm which might be from axelrasmussen@google.com are

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
selftests-vm-add-dev-userfaultfd-test-cases-to-run_vmtestssh.patch