Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

[Andrea Arcangeli <aarcange@redhat.com>]

On Tue, Nov 05, 2019 at 08:06:49AM -0800, Daniel Colascione wrote:
> Sure, but the same argument applies to all the other permission checks
> that we do at open time, not at ioctl time. For better or for worse,
> the DAC-ish model used in most places is that access checks happen at
> file object creation time and anyone who has the FD can perform those
> operations later. Confusing the model by doing *some* permission
> checks at open time and *some* permission checks at usage time makes
> the system harder to understand.

The only case that requires change is if userland requested the
UFFD_FEATURE_EVENT_FORK feature (which AFIK only CRIU does) and that
request is done in the UFFDIO_API call not during the syscall.

Doing the check in the syscall would then break all non privileged
users like if we'd set /proc/sys/vm/unprivileged_userfaultfd to
zero. Qemu for example rightfully never runs with privilege (with a
few exceptions like Kata which should be fixed in fact) and it never
asks for the UFFD_FEATURE_EVENT_FORK feature either.