On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote: > > On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@xxxxxxxxxxxx> 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)
