On Tue, Apr 26, 2022 at 9:46 AM Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> wrote: > > On 4/22/22 3:29 PM, 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 | 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. > > Please keep this sentence in there and rephrase it to indicate how it was > done in the past. > > Also explain here why this new approach is better than the syscall approach > before getting into the below details. Hmm, so the old sentence I think was incorrect already. Notifications of *the faults* aren't delivered and resolved through the syscall. Rather, the syscall just gives you a file descriptor, and then notification / resolution of faults happens though the file descriptor, not through the syscall. So I think it needs to be reworded in any case. I think the overall structure of the doc as-is makes the most sense as well - first explain how this will be used at a very high level, and then go into the details (first how to create a userfaultfd, then how to use it). So, in the end I reworded the "Creating a userfaultfd" section, to cover the two things you mentioned: - Which is the "older" way and which is the "newer" way - What the benefit of the newer way is Hopefully this addresses the comment? I can tweak it more if needed. In any case, thanks for taking a look at this series! > > > +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 > > This is sentence is too short and would look odd. Combine the sentences > so it renders well in the generated doc. Not 100% sure I understood the concern, but I do think it makes sense to move "Vmas are not suitable ..." up into the same paragraph with the other sentence about scalability. I'll do this in v3 as it looks a bit nicer. This leaves the "The userfaultfd, once created, ..." part alone, though. I think s/once opened by invoking the syscall/once created/ is correct, since there are now various ways to create it. I also think that second comma technically should have been there even in the previous version. > > > 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 > > New ioctl? I thought we are moving away from using ioctls? Hmm, looking at alternatives [1] am not sure I see a viable one: We could have defined a new "userfaultfdfs" filesystem, but it seems to me to be overkill for this feature. We could have used a syscall instead and supported fine-grained access control with a new capability, but this approach was rejected [2] generally because we prefer to avoid adding capabilities, and this new capability's scope (just userfaultfd) was considered too narrow. So, I'm not sure of another better way to do this. I suppose one could argue that the dislike of ioctls outweighs the usefulness of this feature, but to me at least the tradeoff seems worth it. :) [1]: https://www.kernel.org/doc/html/latest/driver-api/ioctl.html#alternatives-to-ioctl [2]: https://lkml.org/lkml/2022/2/24/1012 > > > +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 > > +------------------------ > > + > > This will generate doc warn very likley - extend the dashes to the > entire length of the subtitle. I'll fix this in v3. > > > 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 > > =================== > > > > thanks, > -- Shuah
