[PATCH 10/17] prmem: documentation

[Tycho Andersen]

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:
> > > 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;

Do you mean

    p->a = y;

here? I assume the intent is that q isn't writable ever, but that's
the one we have in the structure at rest.

Tycho

> 	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)