Re: [PATCH v3 4/6] userfaultfd: update documentation to describe /dev/userfaultfd

[Mike Rapoport <rppt@kernel.org>]

On Wed, Jun 01, 2022 at 02:09:49PM -0700, Axel Rasmussen wrote:
> 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 | 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

I think 'up' is out of place here. It should be "initializing a
userfaultfd" or "setting 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

Maybe:

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
>  ===================
> -- 
> 2.36.1.255.ge46751e96f-goog
> 
> 

-- 
Sincerely yours,
Mike.