linux-kernel - Re: [PATCH v2 0/6] x86/alternatives: text

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Date:   Wed, 5 Sep 2018 20:56:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 0/6] x86/alternatives: text_poke() fixes

On Sun, Sep 02, 2018 at 10:32:18AM -0700, Nadav Amit wrote:
> This patch-set addresses some issues that were raised in a recent
> correspondence and might affect the security and the correctness of code
> patching. (Note that patching performance is not addressed by this
> patch-set).
> 
> The main issue that the patches deal with is the fact that the fixmap
> PTEs that are used for patching are available for access from other
> cores and might be exploited. They are not even flushed from the TLB in
> remote cores, so the risk is even higher. Address this issue by
> introducing a temporary mm that is only used during patching.
> Unfortunately, due to init ordering, fixmap is still used during
> boot-time patching. Future patches can eliminate the need for it.
> 

Remind me; why are we doing it like this instead of fixing fixmap?
Because while this fixes the text_poke crud, it does leave fixmap
broken.