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@xxxxxxxxxx>
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@xxxxxxxxxx
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Charan Teja Kalla <charante@xxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Dmitry V. Levin <ldv@xxxxxxxxxxxx>
Cc: Gleb Fotengauer-Malinovskiy <glebfm@xxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Nadav Amit <namit@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Zhang Yi <yi.zhang@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxx 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
