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

[Axel Rasmussen <axelrasmussen@google.com>]

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
>