Explain the different ways to create a new userfaultfd, and how access control works for each way. Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> --- 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
