linux-kernel - Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write

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Message-ID: <20170407144526.zxgvpnxt6n7twhqw@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Apr 2017 16:45:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathias Krause <minipli@...glemail.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Mark Rutland <mark.rutland@....com>,
        Hoeun Ryu <hoeun.ryu@...il.com>,
        PaX Team <pageexec@...email.hu>,
        Emese Revfy <re.emese@...il.com>,
        Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement
 __arch_rare_write_begin/unmap()

On Fri, Apr 07, 2017 at 12:51:15PM +0200, Mathias Krause wrote:
> Why that? It allows fast and CPU local modifications of r/o memory.
> OTOH, an approach that needs to fiddle with page table entries
> requires global synchronization to keep the individual TLB states in
> sync. Hmm.. Not that fast, I'd say.

The fixmaps used for kmap_atomic are per-cpu, no global sync required.