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 8:56 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> On Tue, Nov 05, 2019 at 08:39:26AM -0800, Daniel Colascione wrote:
> > 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.
>
> Until now it wasn't clear to me you still wanted to do the permission
> check in UFFDIO_API time, and you only intended to move the
> "measurement" of the capability to the syscall.
>
> So you're suggesting to add more kernel complexity to code pending for
> removal to achieve a theoretically more pure solution in the band-aid
> required to defer the removal of the posix-breaking read
> implementation of the uffd fork feature?

And you're suggesting making a security check work weirdly unlike most
other security checks because you hope it'll get removed one day?
Temporary solutions aren't, and if something goes into the kernel at
all, it's worth getting right. The general rule is that access checks
happen at open time. The kernel has already been bitten by UFFD
exempting itself from the normal rules (e.g., the
read(2)-makes-a-file-descriptor thing) in the name of expediency.
There shouldn't be any more exceptions.