Messages in this thread | | | Date | Tue, 30 Oct 2018 10:58:14 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH 10/17] prmem: documentation |
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On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote: > > On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@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; 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)
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