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 | 40 ++++++++++++++++++-- Documentation/admin-guide/sysctl/vm.rst | 3 ++ 2 files changed, 40 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index 6528036093e1..9bae1acd431f 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 +52,38 @@ 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 by older kernels, 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. + +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; its benefit is in how access to +creating userfaultfds is controlled. + +Access to /dev/userfaultfd is controlled via normal filesystem permissions +(user/group/mode for example), 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). + +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 d7374a1e8ac9..e3a952d1fd35 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. +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 =================== -- 2.36.1.255.ge46751e96f-goog
