Re: [PATCH 0/3] userfaultfd: allow to forbid unprivileged users

Kirill A. Shutemov
2019-03-12T07:49:51Z

On Mon, Mar 11, 2019 at 05:36:58PM +0800, Peter Xu wrote:
> Hi,
>
> (The idea comes from Andrea, and following discussions with Mike and
> other people)
>
> This patchset introduces a new sysctl flag to allow the admin to
> forbid users from using userfaultfd:
>
> $ cat /proc/sys/vm/unprivileged_userfaultfd
> [disabled] enabled kvm

CC linux-api@

This is unusual way to return current value for sysctl. Does it work fine
with sysctl tool?

Have you considered to place the switch into /sys/kernel/mm instead?
I doubt it's the last tunable for userfaultfd. Maybe we should have an
directory for it under /sys/kernel/mm?

> - When set to "disabled", all unprivileged users are forbidden to
> use userfaultfd syscalls.
>
> - When set to "enabled", all users are allowed to use userfaultfd
> syscalls.
>
> - When set to "kvm", all unprivileged users are forbidden to use the
> userfaultfd syscalls, except the user who has permission to open
> /dev/kvm.
>
> This new flag can add one more layer of security to reduce the attack
> surface of the kernel by abusing userfaultfd. Here we grant the
> thread userfaultfd permission by checking against CAP_SYS_PTRACE
> capability. By default, the value is "disabled" which is the most
> strict policy. Distributions can have their own perferred value.
>
> The "kvm" entry is a bit special here only to make sure that existing
> users like QEMU/KVM won't break by this newly introduced flag. What
> we need to do is simply set the "unprivileged_userfaultfd" flag to
> "kvm" here to automatically grant userfaultfd permission for processes
> like QEMU/KVM without extra code to tweak these flags in the admin
> code.
>
> Patch 1: The interface patch to introduce the flag
>
> Patch 2: The KVM related changes to detect opening of /dev/kvm
>
> Patch 3: Apply the flag to userfaultfd syscalls
>
> All comments would be greatly welcomed. Thanks,
>
> Peter Xu (3):
> userfaultfd/sysctl: introduce unprivileged_userfaultfd
> kvm/mm: introduce MMF_USERFAULTFD_ALLOW flag
> userfaultfd: apply unprivileged_userfaultfd check
>
> fs/userfaultfd.c | 121 +++++++++++++++++++++++++++++++++
> include/linux/sched/coredump.h | 1 +
> include/linux/userfaultfd_k.h | 5 ++
> init/Kconfig | 11 +++
> kernel/sysctl.c | 11 +++
> virt/kvm/kvm_main.c | 7 ++
> 6 files changed, 156 insertions(+)
>
> --
> 2.17.1
>

--
Kirill A. Shutemov