@@ -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, 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
@@ -928,6 +928,9 @@ 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
===================
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 | 41 ++++++++++++++++++-- Documentation/admin-guide/sysctl/vm.rst | 3 ++ 2 files changed, 41 insertions(+), 3 deletions(-)