[PATCH v2 4/6] userfaultfd: update documentation to describe /dev/userfaultfd

[Axel Rasmussen <axelrasmussen@google.com>]

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

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 38 ++++++++++++++++++--
 Documentation/admin-guide/sysctl/vm.rst      |  3 ++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 6528036093e1..4c079b5377d4 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:
@@ -39,7 +42,7 @@ 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 +53,37 @@ is a corner case that would currently return ``-EBUSY``).
 API
 ===
 
+Creating a userfaultfd
+----------------------
+
+There are two mechanisms to create a userfaultfd. There are various ways to
+restrict this too, since userfaultfds which handle kernel page faults have
+historically been a useful tool for exploiting the kernel.
+
+The first 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.
+
+Alternatively, userfaultfds can be created by opening /dev/userfaultfd, and
+issuing a USERFAULTFD_IOC_NEW ioctl to this device. Access to this device is
+controlled via normal filesystem permissions (user/group/mode for example) - no
+additional permission (capability/sysctl) is needed to be able to handle kernel
+faults this way. This is useful because it allows e.g. a specific user or group
+to be able to create kernel-fault-handling userfaultfds, without allowing it
+more broadly, or granting more privileges in addition to that particular ability
+(CAP_SYS_PTRACE). In other words, it allows permissions to be minimized.
+
+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
diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f4804ce37c58..8682d5fbc8ea 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -880,6 +880,9 @@ 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
 ===================
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog