From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id 33FDA6B3FCA for ; Mon, 27 Aug 2018 05:39:43 -0400 (EDT) Received: by mail-pf1-f198.google.com with SMTP id j15-v6so11309131pff.12 for ; Mon, 27 Aug 2018 02:39:43 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id q90-v6si15159656pfa.272.2018.08.27.02.39.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 27 Aug 2018 02:39:42 -0700 (PDT) Date: Mon, 27 Aug 2018 18:39:36 +0900 From: Masami Hiramatsu Subject: Re: TLB flushes on fixmap changes Message-Id: <20180827183936.75f4e9bc131cdeac943f44e3@kernel.org> In-Reply-To: <20180827081329.GZ24124@hirez.programming.kicks-ass.net> References: <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> <20180826090958.GT24124@hirez.programming.kicks-ass.net> <20180827120305.01a6f26267c64610cadec5d8@kernel.org> <20180827081329.GZ24124@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Andy Lutomirski , Kees Cook , Nadav Amit , Linus Torvalds , Paolo Bonzini , Jiri Kosina , 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 On Mon, 27 Aug 2018 10:13:29 +0200 Peter Zijlstra wrote: > On Mon, Aug 27, 2018 at 12:03:05PM +0900, Masami Hiramatsu wrote: > > On Sun, 26 Aug 2018 11:09:58 +0200 > > Peter Zijlstra wrote: > > > > FWIW, before text_poke_bp(), text_poke() would only be used from > > > stop_machine, so all the other CPUs would be stuck busy-waiting with > > > IRQs disabled. These days, yeah, that's lots more dodgy, but yes > > > text_mutex should be serializing all that. > > > > I'm still not sure that speculative page-table walk can be done > > over the mutex. Also, if the fixmap area is for aliasing > > pages (which always mapped to memory), what kind of > > security issue can happen? > > So suppose CPU-A is doing the text_poke (let's say through text_poke_bp, > such that other CPUs get to continue with whatever they're doing). > > While at that point, CPU-B gets an interrupt, and the CPU's > branch-trace-buffer for the IRET points to / near our fixmap. Then the > CPU could do a speculative TLB fill based on the BTB value, either > directly or indirectly (through speculative driven fault-ahead) of > whatever is in te fixmap at the time. Hmm, but how "near" is it enough? Since text_poke just map a non executable alias page in fixmap, it is hard to suppose that IRET points there (except for attacker change the IRET address). I see that Intel CPU sometimes speculatively read-ahead the page tables, but in that case, I guess we just need to keep fixmap area away from text area. (Of course, it is hard to estimate how far is enough :( ) Anyway, I agree to introduce new page-table (and kthread?) for fixmap. > Then CPU-A completes the text_poke and only does a local TLB invalidate > on CPU-A, leaving CPU-B with an active translation. > > *FAIL* Ah, I got it. So on CPU-B, it can write-access to fixmap'd pages unless the CPU-B shoot down the full TLB... Thank you, -- Masami Hiramatsu