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

[Schaufler, Casey]

> -----Original Message-----
> From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Sent: Tuesday, July 19, 2022 12:56 PM
> To: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>; Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx>; Dave Hansen
> <dave.hansen@xxxxxxxxxxxxxxx>; Dmitry V . Levin <ldv@xxxxxxxxxxxx>; Gleb
> Fotengauer-Malinovskiy <glebfm@xxxxxxxxxxxx>; Hugh Dickins
> <hughd@xxxxxxxxxx>; Jan Kara <jack@xxxxxxx>; Jonathan Corbet
> <corbet@xxxxxxx>; Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>; Mike
> Kravetz <mike.kravetz@xxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxx>;
> Amit, Nadav <namit@xxxxxxxxxx>; Peter Xu <peterx@xxxxxxxxxx>;
> Shuah Khan <shuah@xxxxxxxxxx>; Suren Baghdasaryan
> <surenb@xxxxxxxxxx>; Vlastimil Babka <vbabka@xxxxxxx>; zhangyi
> <yi.zhang@xxxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>; linux-
> doc@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-
> kselftest@xxxxxxxxxxxxxxx
> 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.
> 
> [1]: https://lore.kernel.org/lkml/686276b9-4530-2045-6bd8-
> 170e5943abe4@xxxxxxxxxxxxxxxx/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