[PATCH 10/17] prmem: documentation

[Matthew Wilcox]

On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
> > On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook at chromium.org> wrote:
> I support the addition of a rare-write mechanism to the upstream kernel.
> And I think that there is only one sane way to implement it: using an
> mm_struct. That mm_struct, just like any sane mm_struct, should only
> differ from init_mm in that it has extra mappings in the *user* region.

I'd like to understand this approach a little better.  In a syscall path,
we run with the user task's mm.  What you're proposing is that when we
want to modify rare data, we switch to rare_mm which contains a
writable mapping to all the kernel data which is rare-write.

So the API might look something like this:

	void *p = rare_alloc(...);	/* writable pointer */
	p->a = x;
	q = rare_protect(p);		/* read-only pointer */

To subsequently modify q,

	p = rare_modify(q);
	q->a = y;
	rare_protect(p);

Under the covers, rare_modify() would switch to the rare_mm and return
(void *)((unsigned long)q + ARCH_RARE_OFFSET).  All of the rare data
would then be modifiable, although you don't have any other pointers
to it.  rare_protect() would switch back to the previous mm and return
(p - ARCH_RARE_OFFSET).

Does this satisfy Igor's requirements?  We wouldn't be able to
copy_to/from_user() while rare_mm was active.  I think that's a feature
though!  It certainly satisfies my interests (kernel code be able to
mark things as dynamically-allocated-and-read-only-after-initialisation)