Re: TLB flushes on fixmap changes

From: Jann Horn <jannh@google.com>
To: Kees Cook <keescook@chromium.org>, Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	mhiramat@kernel.org, Nadav Amit <nadav.amit@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	jkosina@suse.cz, Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	benh@au1.ibm.com, npiggin@gmail.com,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>,
	Adin Scannell <ascannell@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	"David S. Miller" <davem@davemloft.net>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: TLB flushes on fixmap changes
Date: Mon, 27 Aug 2018 00:29:37 +0200	[thread overview]
Message-ID: <CAG48ez0tXJgsuoez3akJr4LW8fDGFFGg_9Mpb=MU7yP_7UbJ=A@mail.gmail.com> (raw)
In-Reply-To: <CAGXu5jJQGiGwQRBGuVrmhQqyUEfRUUSD6WYokc2xezExY9ZNUg@mail.gmail.com>

On Mon, Aug 27, 2018 at 12:11 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Sun, Aug 26, 2018 at 1:15 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Sun, 26 Aug 2018, Andy Lutomirski wrote:
> >> > On Aug 26, 2018, at 9:47 AM, Kees Cook <keescook@chromium.org> wrote:
> >> >> On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@amacapital.net> 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.