Re: [PATCH v4 0/2] Control over userfaultfd kernel-fault handling

[Lokesh Gidra]

On Wed, Sep 23, 2020 at 11:56 PM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
>
> This patch series is split from [1]. The other series enables SELinux
> support for userfaultfd file descriptors so that its creation and
> movement can be controlled.
>
> It has been demonstrated on various occasions that suspending kernel
> code execution for an arbitrary amount of time at any access to
> userspace memory (copy_from_user()/copy_to_user()/...) can be exploited
> to change the intended behavior of the kernel. For instance, handling
> page faults in kernel-mode using userfaultfd has been exploited in [2, 3].
> Likewise, FUSE, which is similar to userfaultfd in this respect, has been
> exploited in [4, 5] for similar outcome.
>
> This small patch series adds a new flag to userfaultfd(2) that allows
> callers to give up the ability to handle kernel-mode faults with the
> resulting UFFD file object. It then adds a 'user-mode only' option to
> the unprivileged_userfaultfd sysctl knob to require unprivileged
> callers to use this new flag.
>
> The purpose of this new interface is to decrease the chance of an
> unprivileged userfaultfd user taking advantage of userfaultfd to
> enhance security vulnerabilities by lengthening the race window in
> kernel code.
>
> [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@xxxxxxxxxx/
> [2] https://duasynt.com/blog/linux-kernel-heap-spray
> [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit
> [4] https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html
> [5] https://bugs.chromium.org/p/project-zero/issues/detail?id=808
>
> Changes since v3:
>
>   - Modified the meaning of value '0' of unprivileged_userfaultfd
>     sysctl knob. Setting this knob to '0' now allows unprivileged users
>     to use userfaultfd, but can handle page faults in user-mode only.
>   - The default value of unprivileged_userfaultfd sysctl knob is changed
>     to '0'.
>
Request reviewers and maintainers to please take a look.

> Changes since v2:
>
>   - Removed 'uffd_flags' and directly used 'UFFD_USER_MODE_ONLY' in
>     userfaultfd().
>
> Changes since v1:
>
>   - Added external references to the threats from allowing unprivileged
>     users to handle page faults from kernel-mode.
>   - Removed the new sysctl knob restricting handling of page
>     faults from kernel-mode, and added an option for the same
>     in the existing 'unprivileged_userfaultfd' knob.
>
> Lokesh Gidra (2):
>   Add UFFD_USER_MODE_ONLY
>   Add user-mode only option to unprivileged_userfaultfd sysctl knob
>
>  Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++-----
>  fs/userfaultfd.c                        | 12 +++++++++---
>  include/uapi/linux/userfaultfd.h        |  9 +++++++++
>  3 files changed, 28 insertions(+), 8 deletions(-)
>
> --
> 2.28.0.681.g6f77f65b4e-goog
>