On Mon, Apr 10, 2017 at 1:13 PM, Kees Cook <keesc...@chromium.org> wrote: > On Sun, Apr 9, 2017 at 1:24 PM, PaX Team <pagee...@freemail.hu> wrote: >> On 7 Apr 2017 at 22:07, Andy Lutomirski wrote: >>> No one has explained how CR0.WP is weaker or slower than my proposal. >> >> you misunderstood, Daniel was talking about your use_mm approach. >> >>> Here's what I'm proposing: >>> >>> At boot, choose a random address A. >> >> what is the threat that a random address defends against? >> >>> Create an mm_struct that has a >>> single VMA starting at A that represents the kernel's rarely-written >>> section. Compute O = (A - VA of rarely-written section). To do a >>> rare write, use_mm() the mm, write to (VA + O), then unuse_mm(). >> >> the problem is that the amount of __read_only data extends beyond vmlinux, >> i.e., this approach won't scale. another problem is that it can't be used >> inside use_mm and switch_mm themselves (no read-only task structs or percpu >> pgd for you ;) and probably several other contexts. > > These are the limitations that concern me: what will we NOT be able to > make read-only as a result of the use_mm() design choice? My RFC > series included a simple case and a constify case, but I did not > include things like making page tables read-only, etc.
If we make page tables read-only, we may need to have multiple levels of rareness. Page table writes aren't all that rare, and I can imagine distros configuring the kernel so that static structs full of function pointers are read-only (IMO that should be the default or even mandatory), but page tables may be a different story. That being said, CR3-twiddling to write to page tables could actually work. Hmm.
