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[209.85.213.178]) by smtp.gmail.com with ESMTPSA id s63-v6sm5800894ywd.63.2018.08.26.15.04.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 Aug 2018 15:04:01 -0700 (PDT) Received: by mail-yb0-f178.google.com with SMTP id o17-v6so5383364yba.2 for ; Sun, 26 Aug 2018 15:04:01 -0700 (PDT) X-Received: by 2002:a25:19c3:: with SMTP id 186-v6mr5773553ybz.410.1535321040528; Sun, 26 Aug 2018 15:04:00 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:a25:2c11:0:0:0:0:0 with HTTP; Sun, 26 Aug 2018 15:03:59 -0700 (PDT) In-Reply-To: References: <20180822153012.173508681@infradead.org> <20180823133958.GA1496@brain-police> <20180824084717.GK24124@hirez.programming.kicks-ass.net> <20180824180438.GS24124@hirez.programming.kicks-ass.net> <56A9902F-44BE-4520-A17C-26650FCC3A11@gmail.com> <9A38D3F4-2F75-401D-8B4D-83A844C9061B@gmail.com> <8E0D8C66-6F21-4890-8984-B6B3082D4CC5@gmail.com> <20180826112341.f77a528763e297cbc36058fa@kernel.org> <952A64F0-90B3-4E2F-B410-7E20BE90D617@amacapital.net> From: Kees Cook Date: Sun, 26 Aug 2018 15:03:59 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: TLB flushes on fixmap changes To: Thomas Gleixner Cc: Andy Lutomirski , Andy Lutomirski , Masami Hiramatsu , Nadav Amit , Linus Torvalds , Paolo Bonzini , Jiri Kosina , Peter Zijlstra , Will Deacon , Benjamin Herrenschmidt , Nick Piggin , "the arch/x86 maintainers" , Borislav Petkov , Rik van Riel , Jann Horn , Adin Scannell , Dave Hansen , Linux Kernel Mailing List , linux-mm , David Miller , Martin Schwidefsky , Michael Ellerman Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 26, 2018 at 1:15 PM, Thomas Gleixner wrote: > On Sun, 26 Aug 2018, Andy Lutomirski wrote: >> > On Aug 26, 2018, at 9:47 AM, Kees Cook wrote: >> >> On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski 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?) -Kees -- Kees Cook Pixel Security