linux-kernel - Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA

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Date:   Fri, 10 Feb 2017 15:39:19 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        Will Deacon <will.deacon@....com>,
        David Windsor <dwindsor@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Howells <dhowells@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures
 through CHECK_DATA_CORRUPTION

On Thu, Feb 9, 2017 at 2:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Feb 08, 2017 at 01:20:26PM -0800, Kees Cook wrote:
>
>> Ooooh, that is intense. And the trampolines (EX_REG_HANDLERs) are all
>> just there to catch whatever register gcc decides to stuff the value
>> into? *cover face* Sure, okay. :)
>
> Right, they shouldn't be big functions, but barring whole program LTO
> there's just no knowing which are unused.
>
>> I wonder how many existing WARN callsites could be repurposed to use this?
>
> At the very least all WARN/BUG instances with trivial @format argument
> that are inlined I think. For example, things like:
>
> static inline some_function()
> {
>         /* ... */
>         WARN(cond, "blah blah blah\n");
>         /* ... */
> }
>
> where the format has no arguments. Here we can out-of-line the printk()
> stuff, which, as is the purpose here, shrinks the size of the inline.

Unless there is some other unholy macros trick, I think we'd need a
separate "WARN_CONST" or something macro to do this (i.e.
WARN_CONST(const, const_str) instead of WARN(cond, fmt, ...)), since
detecting a single-item vararg in a macro is very weird/impossible to
do. Hrmm.

-Kees

-- 
Kees Cook
Pixel Security