On Tue, Oct 30, 2018 at 11:51 AM, Andy Lutomirski [off-list ref] wrote:

quoted

On Oct 30, 2018, at 10:58 AM, Matthew Wilcox [off-list ref] 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);

How about:

rare_write(&q->a, y);

Or, for big writes:

rare_write_copy(&q, local_q);

This avoids a whole ton of issues.  In practice, actually running with a special mm requires preemption disabled as well as some other stuff, which Nadav carefully dealt with.

This is what I had before, yes:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d

It just needs the switch_mm() backend.

-- 
Kees Cook