linux-kernel - Re: [PATCH] rtc: armada38x: add __ro_after_init to armada38x_rtc

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Message-ID: <CAGXu5jLe6RNa6xrefB--w3M+ECRkMMDZWoyAOp5gA1h6W-1vMA@mail.gmail.com>
Date:   Wed, 4 Jan 2017 13:53:55 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Julia Lawall <julia.lawall@...6.fr>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        andrew@...n.ch, Jason Cooper <jason@...edaemon.net>,
        rtc-linux@...glegroups.com,
        Alessandro Zummo <a.zummo@...ertech.it>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        gregory.clement@...e-electrons.com,
        Bhumika Goyal <bhumirks@...il.com>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH] rtc: armada38x: add __ro_after_init to armada38x_rtc_ops

On Wed, Jan 4, 2017 at 5:06 AM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Wed, Jan 04, 2017 at 01:23:41PM +0100, Julia Lawall wrote:
>> Basically, the strategy of the patch is that one may consider it
>> preferable to duplicate the structure for the different alternatives,
>> rather than use __ro_after_init.  Perhaps if the structure were larger,
>> then __ro_after_init would be a better choice?
>
> It depends on not just the size, but how many members need to be
> modified, and obviously whether there are likely to be more than one
> user of the structure as well.
>
> So I'd say __ro_after_init rarely makes sense for an operations
> structure - the only case I can see is:
>
> - a large structure
> - only a small number of elements need to be modified
> - it is only single-use
>
> which is probably quite rare - this one falls into two out of those
> three.
>
> There's another consideration (imho) too - we may wish, at a later
> date, to make .text and .rodata both read-only from the start of the
> kernel to harden the kernel against possibly init-time exploitation.
> (Think about a buggy built-in driver with emulated hardware - much the
> same problem that Kees is trying to address in one of his recent patch
> sets but with hotplugged hardware while a screen-saver is active.)
> Having function pointers in .rodata rather than the ro-after-init
> section would provide better protection.

Agreed: I'd much prefer things just be const. :) As to my confusing
question, I hadn't looked at how where the pointers to the structure
was being stored, so I was just asking if it, too, could be const,
which it can't, and that's fine here.

-Kees

-- 
Kees Cook
Nexus Security