On Mon, Aug 08, 2022 at 10:56:13AM -0700, Axel Rasmussen wrote:
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..a76c9dc1865b 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, then either
I think "then" is excessive here ^
- 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
Maybe:
..., is by opening /dev/userfaultfd and issuing USERFAULTFD_IOC_NEW ioctl to it.
+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 diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index f74f722ad702..b3e40b42e1b3 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -927,6 +927,9 @@ 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 =================== -- 2.37.1.559.g78731f0fdb-goog