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From:   Thomas Garnier <thgarnie@chromium.org>
Date:   Thu, 29 Aug 2019 12:55:00 -0700
Message-ID: <CAJcbSZETvvQYmh6U_Oauptdsrp-emmSG_QsAZzKLv+0-b2Yxig@mail.gmail.com>
Subject: Re: [PATCH v9 00/11] x86: PIE support to extend KASLR randomization
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Borislav Petkov <bp@alien8.de>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
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        "David S. Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
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        Thomas Hellstrom <thellstrom@vmware.com>,
        "VMware, Inc." <pv-drivers@vmware.com>,
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On Tue, Aug 6, 2019 at 8:51 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Aug 06, 2019 at 05:43:47PM +0200, Borislav Petkov wrote:
> > On Tue, Jul 30, 2019 at 12:12:44PM -0700, Thomas Garnier wrote:
> > > These patches make some of the changes necessary to build the kernel as
> > > Position Independent Executable (PIE) on x86_64. Another patchset will
> > > add the PIE option and larger architecture changes.
> >
> > Yeah, about this: do we have a longer writeup about the actual benefits
> > of all this and why we should take this all? After all, after looking
> > at the first couple of asm patches, it is posing restrictions to how
> > we deal with virtual addresses in asm (only RIP-relative addressing in
> > 64-bit mode, MOVs with 64-bit immediates, etc, for example) and I'm
> > willing to bet money that some future unrelated change will break PIE
> > sooner or later.

The goal is being able to extend the range of addresses where the
kernel can be placed with KASLR. I will look at clarifying that in the
future.

>
> Possibly objtool can help here; it should be possible to teach it about
> these rules, and then it will yell when violated. That should avoid
> regressions.
>

I will look into that as well.