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

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Message-ID: <CALCETrW7nN8Ja=r9k4ANpekKovC_wsYrGrge1WNoGsAjZZvqHQ@mail.gmail.com>
Date:   Wed, 5 Apr 2017 16:57:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Mark Rutland <mark.rutland@....com>,
        Andy Lutomirski <luto@...nel.org>,
        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: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

On Wed, Mar 29, 2017 at 6:41 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Mar 29, 2017 at 3:38 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Wed, Mar 29, 2017 at 11:15 AM, Kees Cook <keescook@...omium.org> wrote:
>>> Based on PaX's x86 pax_{open,close}_kernel() implementation, this
>>> allows HAVE_ARCH_RARE_WRITE to work on x86.
>>>
>>
>>> +
>>> +static __always_inline unsigned long __arch_rare_write_begin(void)
>>> +{
>>> +       unsigned long cr0;
>>> +
>>> +       preempt_disable();
>>
>> This looks wrong.  DEBUG_LOCKS_WARN_ON(!irqs_disabled()) would work,
>> as would local_irq_disable().  There's no way that just disabling
>> preemption is enough.
>>
>> (Also, how does this interact with perf nmis?)
>
> Do you mean preempt_disable() isn't strong enough here? I'm open to
> suggestions. The goal would be to make sure nothing between _begin and
> _end would get executed without interruption...
>

Sorry for the very slow response.

preempt_disable() isn't strong enough to prevent interrupts, and an
interrupt here would run with WP off, causing unknown havoc.  I tend
to think that the caller should be responsible for turning off
interrupts.

--Andy