On Fri, Oct 26, 2018 at 11:46:28AM +0100, Kees Cook wrote: > On Fri, Oct 26, 2018 at 10:26 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > I still don't really understand the whole write-rare thing; how does it > > really help? If we can write in kernel memory, we can write to > > page-tables too. > One aspect of hardening the kernel against attack is reducing the > internal attack surface. Not all flaws are created equal, so there is > variation in what limitations an attacker may have when exploiting > flaws (not many flaws end up being a fully controlled "write anything, > anywhere, at any time"). By making the more sensitive data structures > of the kernel read-only, we reduce the risk of an attacker finding a > path to manipulating the kernel's behavior in a significant way. > > Examples of typical sensitive targets are function pointers, security > policy, and page tables. Having these "read only at rest" makes them > much harder to control by an attacker using memory integrity flaws. Because 'write-anywhere' exploits are easier than (and the typical first step to) arbitrary code execution thingies? > The "write rarely" name itself may not sufficiently describe what is > wanted either (I'll take the blame for the inaccurate name), so I'm > open to new ideas there. The implementation requirements for the > "sensitive data read-only at rest" feature are rather tricky: > > - allow writes only from specific places in the kernel > - keep those locations inline to avoid making them trivial ROP targets > - keep the writeability window open only to a single uninterruptable CPU The current patch set does not achieve that because it uses a global address space for the alias mapping (vmap) which is equally accessible from all CPUs. > - fast enough to deal with page table updates The proposed implementation needs page-tables for the alias; I don't see how you could ever do R/O page-tables when you need page-tables to modify your page-tables. And this is entirely irrespective of performance. > The proposal I made a while back only covered .data things (and used > x86-specific features). Oh, right, that CR0.WP stuff. > Igor's proposal builds on this by including a > way to do this with dynamic allocation too, which greatly expands the > scope of structures that can be protected. Given that the x86-only > method of write-window creation was firmly rejected, this is a new > proposal for how to do it (vmap window). Using switch_mm() has also > been suggested, etc. Right... /me goes find the patches we did for text_poke. Hmm, those never seem to have made it: https://lkml.kernel.org/r/20180902173224.30606-1-namit@xxxxxxxxxx like that. That approach will in fact work and not be a completely broken mess like this thing. > We need to find a good way to do the write-windowing that works well > for static and dynamic structures _and_ for the page tables... this > continues to be tricky. > > Making it resilient against ROP-style targets makes it difficult to > deal with certain data structures (like list manipulation). In my > earlier RFC, I tried to provide enough examples of where this could > get used to let people see some of the complexity[1]. Igor's series > expands this to even more examples using dynamic allocation. Doing 2 CR3 writes for 'every' WR write doesn't seem like it would be fast enough for much of anything. And I don't suppose we can take the WP fault and then fix up from there, because if we're doing R/O page-tables, that'll incrase the fault depth and we'll double fault all the time, and tripple fault where we currently double fault. And we all know how _awesome_ tripple faults are. But duplicating (and wrapping in gunk) whole APIs is just not going to work.
