[PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Axel Rasmussen <axelrasmussen@google.com>]

This series is based on torvalds/master.

The series is split up like so:
- Patch 1 is a simple fixup which we should take in any case (even by itself).
- Patches 2-6 add the feature, configurable selftest support, and docs.

Why not ...?
============

- Why not /proc/[pid]/userfaultfd? The proposed use case for this is for one
  process to open a userfaultfd which can intercept another process' page
  faults. This seems to me like exactly what CAP_SYS_PTRACE is for, though, so I
  think this use case can simply use a syscall without the powers CAP_SYS_PTRACE
  grants being "too much".

- Why not use a syscall? Access to syscalls is generally controlled by
  capabilities. We don't have a capability which is used for userfaultfd access
  without also granting more / other permissions as well, and adding a new
  capability was rejected [1].

    - It's possible a LSM could be used to control access instead. I suspect
      adding a brand new one just for this would be rejected, but I think some
      existing ones like SELinux can be used to filter syscall access. Enabling
      SELinux for large production deployments which don't already use it is
      likely to be a huge undertaking though, and I don't think this use case by
      itself is enough to motivate that kind of architectural change.

Changelog
=========

v3->v4:
  - Picked up an Acked-by on 5/5.
  - Updated cover letter to cover "why not ...".
  - Refactored userfaultfd_allowed() into userfaultfd_syscall_allowed(). [Peter]
  - Removed obsolete comment from a previous version. [Peter]
  - Refactored userfaultfd_open() in selftest. [Peter]
  - Reworded admin-guide documentation. [Mike, Peter]
  - Squashed 2 commits adding /dev/userfaultfd to selftest and making selftest
    configurable. [Peter]
  - Added "syscall" test modifier (the default behavior) to selftest. [Peter]

v2->v3:
  - Rebased onto linux-next/akpm-base, in order to be based on top of the
    run_vmtests.sh refactor which was merged previously.
  - Picked up some Reviewed-by's.
  - Fixed ioctl definition (_IO instead of _IOWR), and stopped using
    compat_ptr_ioctl since it is unneeded for ioctls which don't take a pointer.
  - Removed the "handle_kernel_faults" bool, simplifying the code. The result is
    logically equivalent, but simpler.
  - Fixed userfaultfd selftest so it returns KSFT_SKIP appropriately.
  - Reworded documentation per Shuah's feedback on v2.
  - Improved example usage for userfaultfd selftest.

v1->v2:
  - Add documentation update.
  - Test *both* userfaultfd(2) and /dev/userfaultfd via the selftest.

[1]: https://lore.kernel.org/lkml/686276b9-4530-2045-6bd8-170e5943abe4@schaufler-ca.com/T/

Axel Rasmussen (5):
  selftests: vm: add hugetlb_shared userfaultfd test to run_vmtests.sh
  userfaultfd: add /dev/userfaultfd for fine grained access control
  userfaultfd: selftests: modify selftest to use /dev/userfaultfd
  userfaultfd: update documentation to describe /dev/userfaultfd
  selftests: vm: add /dev/userfaultfd test cases to run_vmtests.sh

 Documentation/admin-guide/mm/userfaultfd.rst | 41 +++++++++++-
 Documentation/admin-guide/sysctl/vm.rst      |  3 +
 fs/userfaultfd.c                             | 69 ++++++++++++++++----
 include/uapi/linux/userfaultfd.h             |  4 ++
 tools/testing/selftests/vm/run_vmtests.sh    | 11 +++-
 tools/testing/selftests/vm/userfaultfd.c     | 69 +++++++++++++++++---
 6 files changed, 169 insertions(+), 28 deletions(-)

--
2.37.0.170.g444d1eabd0-goog