-----Original Message----- From: Axel Rasmussen axelrasmussen@google.com Sent: Tuesday, July 19, 2022 12:56 PM To: Alexander Viro viro@zeniv.linux.org.uk; Andrew Morton akpm@linux-foundation.org; Dave Hansen dave.hansen@linux.intel.com; Dmitry V . Levin ldv@altlinux.org; Gleb Fotengauer-Malinovskiy glebfm@altlinux.org; Hugh Dickins hughd@google.com; Jan Kara jack@suse.cz; Jonathan Corbet corbet@lwn.net; Mel Gorman mgorman@techsingularity.net; Mike Kravetz mike.kravetz@oracle.com; Mike Rapoport rppt@kernel.org; Amit, Nadav namit@vmware.com; Peter Xu peterx@redhat.com; Shuah Khan shuah@kernel.org; Suren Baghdasaryan surenb@google.com; Vlastimil Babka vbabka@suse.cz; zhangyi yi.zhang@huawei.com Cc: Axel Rasmussen axelrasmussen@google.com; linux- doc@vger.kernel.org; linux-fsdevel@vger.kernel.org; linux- kernel@vger.kernel.org; linux-mm@kvack.org; linux- kselftest@vger.kernel.org Subject: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

I assume that leaving the LSM mailing list off of the CC is purely accidental. Please, please include us in the next round.

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,

You won't know if you don't ask.

  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.

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