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@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 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.
