On Tue, Oct 30, 2018 at 10:58:14AM -0700, Matthew Wilcox wrote:

On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:

quoted

quoted

On Oct 30, 2018, at 9:37 AM, Kees Cook [off-list ref] 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);

Why would you have rare_alloc() imply rare_modify() ? Would you have the
allocator meta data inside the rare section?