Re: [PATCH 2/4] mm: introduce external memory hinting API

[Daniel Colascione]

On Mon, Jan 13, 2020 at 12:42 PM Christian Brauner
<christian.brauner@xxxxxxxxxx> wrote:
>
> On Mon, Jan 13, 2020 at 11:27:03AM -0800, Daniel Colascione wrote:
> > On Mon, Jan 13, 2020 at 11:10 AM Christian Brauner
> > <christian.brauner@xxxxxxxxxx> wrote:
> > > This does not
> > > affect the permission checking you're performing here.
> >
> > Pidfds-as-capabilities sounds like a good change. Can you clarify what
> > you mean here though? Do you mean that in order to perform some
> > process-directed operation X on process Y, the pidfd passed to X must
> > have been opened with PIDFD_CAP_X *and* the process *using* the pidfds
> > must be able to perform operation X on process Y? Or do pidfds in this
> > model "carry" permissions in the same way that an ordinary file
> > descriptor "carries" the ability to write to a file if it was opened
> > with O_WRONLY even if the FD is passed to a process that couldn't
> > otherwise write to that file? Right now, pidfds are identity-only and
> > always rely on the caller's permissions. I like the capability bit
> > model because it makes pidfds more consistent with other file
> > descriptors and enabled delegation of capabilities across the system.
>
> I'm going back and forth on this. My initial implementation has it that
> you'd need both, PIDFD_FLAG/CAP_X and the process using the pidfd must
> be able to perform the operation X on process Y. The alternative becomes
> tricky for e.g. anything that requires ptrace_may_access() permissions
> such as getting an fd out from another task based on its pidfd and so
> on.

I think the alternative is necessary though. What's the point of the
pidfd capability bits if they don't grant access? If I have a pidfd
for Y that doesn't let me do operation X, but I have ambient authority
to do Y anyway, then I can just make my own pidfd for Y and then use
that new pidfd to do X. AFAICT, pidfd capabilities only do something
when they replace ptrace_may_access and friends for access control.
Otherwise, they seem purely advisory. Am I missing something?