On Wed, Oct 24, 2018 at 05:03:01PM +0300, Igor Stoppa wrote: > On 24/10/18 14:37, Mathieu Desnoyers wrote: > > Also, is it the right approach to duplicate existing APIs, or should we > > rather hook into page fault handlers and let the kernel do those "shadow" > > mappings under the hood ? > > This question is probably a good candidate for the small Q&A section I have > in the 00/17. > > > > Adding a new GFP flags for dynamic allocation, and a macro mapping to > > a section attribute might suffice for allocation or definition of such > > mostly-read-only/seldom-updated data. > > I think what you are proposing makes sense from a pure hardening standpoint. > From a more defensive one, I'd rather minimise the chances of giving a free > pass to an attacker. > > Maybe there is a better implementation of this, than what I have in mind. > But, based on my current understanding of what you are describing, there > would be few issues: > > 1) where would the pool go? The pool is a way to manage multiple vmas and > express common property they share. Even before a vma is associated to the > pool. > > 2) there would be more code that can seamlessly deal with both protected and > regular data. Based on what? Some parameter, I suppose. > That parameter would be the new target. > If the code is "duplicated", as you say, the actual differences are baked in > at compile time. The "duplication" would also allow to have always inlined > functions for write-rare and leave more freedom to the compiler for their > non-protected version. > > Besides, I think the separate wr version also makes it very clear, to the > user of the API, that there will be a price to pay, in terms of performance. > The more seamlessly alternative might make this price less obvious. What about something in the middle, where we move list to list_impl.h, and add a few macros where you have list_set_prev() in prlist now, so we could do, // prlist.h #define list_set_next(head, next) wr_ptr(&head->next, next) #define list_set_prev(head, prev) wr_ptr(&head->prev, prev) #include <linux/list_impl.h> // list.h #define list_set_next(next) (head->next = next) #define list_set_next(prev) (head->prev = prev) #include <linux/list_impl.h> I wonder then if you can get rid of some of the type punning too? It's not clear exactly why that's necessary from the series, but perhaps I'm missing something obvious :) I also wonder how much the actual differences being baked in at compile time makes. Most (all?) of this code is inlined. Cheers, Tycho
