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    SubjectRe: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control
    On Wed, Jul 20, 2022 at 3:16 PM Schaufler, Casey
    <casey.schaufler@intel.com> wrote:
    >
    > > -----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.

    Honestly it just hadn't occurred to me, but I'm more than happy to CC
    it on future revisions.

    >
    > >
    > > 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.

    Fair enough - I wonder if MM folks (Andrew, Peter, Nadav especially)
    would find that approach more palatable than /proc/[pid]/userfaultfd?
    Would it make sense from our perspective to propose a userfaultfd- or
    MM-specific LSM for controlling access to certain features?

    I remember +Andrea saying Red Hat was also interested in some kind of
    access control mechanism like this. Would one or the other approach be
    more convenient for you?

    >
    > > 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
    >

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