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

[Daniel Colascione <dancol@google.com>]

On Tue, Nov 5, 2019 at 8:33 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> 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.

I'm not suggesting that we fail userfaultfd(2) without CAP_SYS_PTRACE.
That would, as you point out, break things. I'm talking about
recording *whether* we had CAP_SYS_PTRACE in an internal flag in the
uffd context when we create the thing --- and then, at ioctl time,
checking that flag, not the caller's CAP_SYS_PTRACE, to see whether
UFFD_FEATURE_EVENT_FORK should be made available. This way, the
security check hinges on whether the caller *at create time* was
privileged.