linux-kernel - Re: [PATCH v2 7/9] x86: Add support for function granular KASLR

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Message-ID: <7c8685090dd36ab0175ae91d1421f4cd7fb6aff0.camel@linux.intel.com>
Date:   Thu, 21 May 2020 14:42:56 -0700
From:   Kristen Carlson Accardi <kristen@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        Jonathan Corbet <corbet@....net>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        rick.p.edgecombe@...el.com, Tony Luck <tony.luck@...el.com>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 7/9] x86: Add support for function granular KASLR

Hi Kees,
Thanks for your review - I will incorporate what I can into v3, or
explain why not once I give it a try :).

On Thu, 2020-05-21 at 14:08 -0700, Kees Cook wrote:
> > 
<snip>

> On Thu, May 21, 2020 at 09:56:38AM -0700, Kristen Carlson Accardi
> wrote:
> > +	/*
> > +	 * sometimes we are updating a relative offset that would
> > +	 * normally be relative to the next instruction (such as a
> > call).
> > +	 * In this case to calculate the target, you need to add 32bits
> > to
> > +	 * the pc to get the next instruction value. However, sometimes
> > +	 * targets are just data that was stored in a table such as
> > ksymtab
> > +	 * or cpu alternatives. In this case our target is not relative
> > to
> > +	 * the next instruction.
> > +	 */
> 
> Excellent and scary comment. ;) Was this found by trial and error?
> That
> sounds "fun" to debug. :P

This did suck to debug. Thank goodness for debugging with gdb in a VM.
As you know, I had previously had a patch to use a prand to be able to
retain the same layout across boots, and that came in handy here. While
we decided to not submit this functionality with this initial merge
attempt, I will add it on in the future as it does make debugging much
easier when you can reliably duplicate failure modes.