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

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Message-ID: <CA+DvKQKTHBdyQ0_Kn0dpZzAo1g3yBt-OGK309K+UN6upq-5FAQ@mail.gmail.com>
Date:   Sat, 8 Apr 2017 00:12:53 -0400
From:   Daniel Micay <danielmicay@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Mathias Krause <minipli@...glemail.com>,
        "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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

> I probably chose the wrong name for this feature (write rarely).
> That's _usually_ true, but "sensitive_write()" was getting rather
> long. The things that we need to protect with this are certainly stuff
> that doesn't get much writing, but some things are just plain
> sensitive (like page tables) and we should still try to be as fast as
> possible with them.

Not too late to rename it. Scoped write? I think it makes change to
use a different API than PaX for portability too, but not a different
x86 implementation. It's quite important to limit the writes to the
calling thread and it needs to perform well to be introduced widely.

> I'm all for a general case for the infrastructure (as Andy and Mark
> has mentioned), but I don't want to get into the situation where
> people start refusing to use it because it's "too slow" (for example,
> see refcount_t vs net-dev right now).

Meanwhile, the PaX implementation has improved to avoid the issues
that were brought up while only introducing a single always-predicted
(due to code placement) branch on the overflow flag. That seems to
have gone unnoticed upstream, where there's now a much slower
implementation that's not more secure, and is blocked from
introduction in areas where it's most needed based on the performance.
Not to mention that it's opt-in... which is never going to work.