On Wed, Oct 31, 2018 at 10:36:59AM +0100, Peter Zijlstra wrote: > 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? Normally when I allocate some memory I need to initialise it before doing anything else with it ;-) I mean, you could do: ro = rare_alloc(..); rare = rare_modify(ro); rare->a = x; rare_protect(rare); but that's more typing.
