linux-kernel - Re: [RFC PATCH 08/11] x86: Add support for finer grained KASLR

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Date:   Wed, 5 Feb 2020 17:17:11 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH 08/11] x86: Add support for finer grained KASLR

On Wed, Feb 5, 2020 at 2:39 PM Kristen Carlson Accardi
<kristen@...ux.intel.com> wrote:
>
> At boot time, find all the function sections that have separate .text
> sections, shuffle them, and then copy them to new locations. Adjust
> any relocations accordingly.
>

> +       sort(base, num_syms, sizeof(int), kallsyms_cmp, kallsyms_swp);

Hah, here's a huge bottleneck.  Unless you are severely
memory-constrained, never do a sort with an expensive swap function
like this.  Instead allocate an array of indices that starts out as
[0, 1, 2, ...].  Sort *that* where the swap function just swaps the
indices.  Then use the sorted list of indices to permute the actual
data.  The result is exactly one expensive swap per item instead of
one expensive swap per swap.

--Andy