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

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Date:   Tue, 7 Feb 2017 09:34:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "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: [PATCH 4/4] refcount: Report failures through
 CHECK_DATA_CORRUPTION

On Mon, Feb 06, 2017 at 08:54:38AM -0800, Kees Cook wrote:
> >
> > Like I wrote, ideally we'd end up using something like the x86 exception
> > table with a custom handler. Just no idea how to pull that off without
> > doing a full blown arch specific implementation, so I didn't go there
> > quite yet.
> 
> I haven't spent much time looking at the extable stuff. (Though
> coincidentally, I was poking at it for x86's test_nx stuff...) I
> thought there was a way to build arch-agnostic extables already?
> kernel/extable.c is unconditionally built-in, for example.

That doesn't seem to be of much use. It only contains section sort and
search functions.

Another problem for generic code would be to figure out what register
the relevant variable would live in at the time of exception. Here its
'obviously' EAX because that's what cmpxchg requires, but in generic
you'd need a means of querying GCC's register allocator at the exception
point and somehow using that information for the generation of the
exception handler.

AFAIK that's not something GCC can do.

> > and the handler is shared between all instances and can be as big and
> > fancy as we'd like.
> 
> I'll dig a bit to see what I can build. Can you add the lkdtm tests to
> the series, though? That should be fine as-is.

Yes, I did. I also did the 's/--help--/help/' and 's/#if/#ifdef/' thing.

Thanks!