[PATCH v6 4/5] 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.

Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 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