On Mon, Aug 08, 2022 at 10:56:11AM -0700, Axel Rasmussen wrote:

Historically, it has been shown that intercepting kernel faults with
userfaultfd (thereby forcing the kernel to wait for an arbitrary amount
of time) can be exploited, or at least can make some kinds of exploits
easier. So, in 37cd0575b8 "userfaultfd: add UFFD_USER_MODE_ONLY" we
changed things so, in order for kernel faults to be handled by
userfaultfd, either the process needs CAP_SYS_PTRACE, or this sysctl
must be configured so that any unprivileged user can do it.

In a typical implementation of a hypervisor with live migration (take
QEMU/KVM as one such example), we do indeed need to be able to handle
kernel faults. But, both options above are less than ideal:

- Toggling the sysctl increases attack surface by allowing any
  unprivileged user to do it.

- Granting the live migration process CAP_SYS_PTRACE gives it this
  ability, but *also* the ability to "observe and control the
  execution of another process [...], and examine and change [its]
  memory and registers" (from ptrace(2)). This isn't something we need
  or want to be able to do, so granting this permission violates the
  "principle of least privilege".

This is all a long winded way to say: we want a more fine-grained way to
grant access to userfaultfd, without granting other additional
permissions at the same time.

To achieve this, add a /dev/userfaultfd misc device. This device
provides an alternative to the userfaultfd(2) syscall for the creation
of new userfaultfds. The idea is, any userfaultfds created this way will
be able to handle kernel faults, without the caller having any special
capabilities. Access to this mechanism is instead restricted using e.g.
standard filesystem permissions.

Acked-by: Nadav Amit <redacted>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Acked-by: Mike Rapoport <redacted>

---
 fs/userfaultfd.c                 | 73 +++++++++++++++++++++++++-------
 include/uapi/linux/userfaultfd.h |  4 ++
 2 files changed, 61 insertions(+), 16 deletions(-)

-- 
Sincerely yours,
Mike.