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