[PATCH v5 4/5] userfaultfd: update documentation to describe /dev/userfaultfd

[Mike Rapoport]

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 at redhat.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen at 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
> 

-- 
Sincerely yours,
Mike.