On Mon, Aug 27, 2018 at 12:11 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Sun, Aug 26, 2018 at 1:15 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > On Sun, 26 Aug 2018, Andy Lutomirski wrote: > >> > 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: > >> >>> 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. > > > > That's exactly the problem I have with CR0. It leaves everything and some > > more writeable for any code which can interrupt that section. > > I thought the point was that the implementation I suggested was > NMI-proof? (And in reading Documentation/preempt-locking.txt it sounds > like disabling interrupts is redundant to preempt_disable()? But I > don't understand how; it looks like the preempt stuff is advisory?) Where are you dealing with NMIs? local_irq_disable() disables the interrupt flag, but Non-Maskable Interrupts can still come in. As far as I know, the only way to block those is to artificially generate an NMI yourself (Xen does that sometimes). Otherwise, you have to twiddle CR0.WP in the NMI handler entry/exit code.
