Re: [PATCH 0/2] Control over userfaultfd kernel-fault handling

[Lokesh Gidra]

On Fri, Jul 24, 2020 at 7:01 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Wed, Apr 22, 2020 at 05:26:30PM -0700, Daniel Colascione wrote:
> > This small patch series adds a new flag to userfaultfd(2) that allows
> > callers to give up the ability to handle user-mode faults with the
> > resulting UFFD file object. In then add a new sysctl to require
> > unprivileged callers to use this new flag.
> >
> > The purpose of this new interface is to decrease the change of an
> > unprivileged userfaultfd user taking advantage of userfaultfd to
> > enhance security vulnerabilities by lengthening the race window in
> > kernel code.
>
> There are other ways to lengthen the race window, such as madvise
> MADV_DONTNEED, mmap of fuse files ...
> Could the patchset commit log include some discussion about
> why these are not the concern please?
>
> Multiple subsystems including vhost have come to rely on
> copy from/to user behaving identically to userspace access.
>
> Could the patchset please include discussion on what effect blocking
> these will have? E.g. I guess Android doesn't use vhost right now.
> Will it want to do it to run VMs in 2021?
>
> Thanks!
>
> > This patch series is split from [1].
> >
> > [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@xxxxxxxxxx/
>
> So in that series, Kees said:
> https://lore.kernel.org/lkml/202002112332.BE71455@keescook/#t
>
> What is the threat being solved? (I understand the threat, but detailing
>   it in the commit log is important for people who don't know it.)
>

Adding Android security folks, Nick and Jeff, to answer.

> Could you pls do that?
>
> > Daniel Colascione (2):
> >   Add UFFD_USER_MODE_ONLY
> >   Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only
> >
> >  Documentation/admin-guide/sysctl/vm.rst | 13 +++++++++++++
> >  fs/userfaultfd.c                        | 18 ++++++++++++++++--
> >  include/linux/userfaultfd_k.h           |  1 +
> >  include/uapi/linux/userfaultfd.h        |  9 +++++++++
> >  kernel/sysctl.c                         |  9 +++++++++
> >  5 files changed, 48 insertions(+), 2 deletions(-)
> >
> > --
> > 2.26.2.303.gf8c07b1a785-goog
> >
>