From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 5D80B6B3D2B for ; Sun, 26 Aug 2018 18:30:06 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id l14-v6so13156942oii.9 for ; Sun, 26 Aug 2018 15:30:06 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id e126-v6sor7072876oig.186.2018.08.26.15.30.05 for (Google Transport Security); Sun, 26 Aug 2018 15:30:05 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: From: Jann Horn Date: Mon, 27 Aug 2018 00:29:37 +0200 Message-ID: Subject: Re: TLB flushes on fixmap changes Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Kees Cook , Andy Lutomirski Cc: Thomas Gleixner , Andy Lutomirski , mhiramat@kernel.org, Nadav Amit , Linus Torvalds , Paolo Bonzini , jkosina@suse.cz, Peter Zijlstra , Will Deacon , benh@au1.ibm.com, npiggin@gmail.com, the arch/x86 maintainers , Borislav Petkov , Rik van Riel , Adin Scannell , Dave Hansen , kernel list , Linux-MM , "David S. Miller" , Martin Schwidefsky , Michael Ellerman On Mon, Aug 27, 2018 at 12:11 AM Kees Cook wrote: > > 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?) 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.