> On Aug 26, 2018, at 9:47 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > >> On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> >> >>>> On Aug 25, 2018, at 9:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>>> >>>>> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>>>> On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: >>>>> On Fri, 24 Aug 2018 21:23:26 -0700 >>>>> Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>>>>> Couldn't text_poke() use kmap_atomic()? Or, even better, just change CR3? >>>>> >>>>> No, since kmap_atomic() is only for x86_32 and highmem support kernel. >>>>> In x86-64, it seems that returns just a page address. That is not >>>>> good for text_poke, since it needs to make a writable alias for RO >>>>> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache? >>>>> >>>> >>>> I just re-read text_poke(). It's, um, horrible. Not only is the >>>> implementation overcomplicated and probably buggy, but it's SLOOOOOW. >>>> It's totally the wrong API -- poking one instruction at a time >>>> basically can't be efficient on x86. The API should either poke lots >>>> of instructions at once or should be text_poke_begin(); ...; >>>> text_poke_end();. >>>> >>>> Anyway, the attached patch seems to boot. Linus, Kees, etc: is this >>>> too scary of an approach? With the patch applied, text_poke() is a >>>> fantastic exploit target. On the other hand, even without the patch >>>> applied, text_poke() is every bit as juicy. >>> >>> I tried to convince Ingo to use this method for doing "write rarely" >>> and he soundly rejected it. :) I've always liked this because AFAICT, >>> it's local to the CPU. I had proposed it in >>> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d >> >> Ingo, can you clarify why you hate it? I personally would rather use CR3, but CR0 seems like a fine first step, at least for text_poke. > > Sorry, it looks like it was tglx, not Ingo: > > https://lkml.kernel.org/r/alpine.DEB.2.20.1704071048360.1716@nanos > > This thread is long, and one thing that I think went unanswered was > "why do we want this to be fast?" the answer is: for doing page table > updates. Page tables are becoming a bigger target for attacks now, and > it's be nice if they could stay read-only unless they're getting > updated (with something like this). > > It kind of sounds like tglx would prefer the CR3 approach. And indeed my patch has a serious problem wrt the NMI code.
