linux-kernel - Re: [RFC] arm64: support HAVE_ARCH_RARE

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Date:   Sat, 4 Mar 2017 14:56:44 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Emese Revfy <re.emese@...il.com>,
        Russell King <linux@...linux.org.uk>,
        PaX Team <pageexec@...email.hu>, X86 ML <x86@...nel.org>
Subject: Re: [RFC] arm64: support HAVE_ARCH_RARE_WRITE


> On Mar 4, 2017, at 5:50 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 
>> On Thu, Mar 2, 2017 at 7:00 AM, Hoeun Ryu <hoeun.ryu@...il.com> wrote:
>> +unsigned long __rare_write_rw_alias_start = TASK_SIZE_64 / 4;
>> +
>> +__always_inline unsigned long __arch_rare_write_map(void)
>> +{
>> +       struct mm_struct *mm = &rare_write_mm;
>> +
>> +       preempt_disable();
>> +
>> +       __switch_mm(mm);
> 
> ...
> 
>> +__always_inline unsigned long __arch_rare_write_unmap(void)
>> +{
>> +       struct mm_struct *mm = current->active_mm;
>> +
>> +       __switch_mm(mm);
>> +
> 
> This reminds me: this code imposes constraints on the context in which
> it's called.  I'd advise making it very explicit, asserting
> correctness, and putting the onus on the caller to set things up.  For
> example:
> 
> DEBUG_LOCKS_WARN_ON(preemptible() || in_interrupt() || in_nmi());
> 

OK. I will add some onus in the next version.

> in both the map and unmap functions, along with getting rid of the
> preempt_disable().  I don't think we want the preempt-disabledness to
> depend on the arch.  The generic non-arch rare_write helpers can do
> the preempt_disable().
> 

I think I can fix this in the next version when Kees send the next version of RFC.

> This code also won't work if the mm is wacky when called.  On x86, we could do:
> 
> DEBUG_LOCKS_WARN_ON(read_cr3() != current->active_mm->pgd);
> 
> or similar (since that surely doesn't compile as is).
> 
> --Andy

Thank you for the review.