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Date:   Fri, 3 Nov 2017 20:09:42 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [PATCH 13/15] arm64: add a workaround for GNU gold with ARM64_MODULE_PLTS

On 3 November 2017 at 17:12, Sami Tolvanen <samitolvanen@...gle.com> wrote:
> CONFIG_CLANG_LTO depends on GNU gold and due to a known bug, the
> linker crashes when ARM64_MODULE_PLTS is enabled:
>
>   https://sourceware.org/bugzilla/show_bug.cgi?id=14592
>
> To work around the problem, this change:
>
>   1) Enables ARM64_MODULE_PLTS only when it's needed, i.e. when
>      RANDOMIZE_MODULE_REGION_FULL is selected,
>

This is not true. Even when RANDOMIZE_MODULE_REGION_FULL is not
enabled, we may need module PLTs.

When the module region is randomized, it shares its chunk of vmalloc
space with other users, and so there is a [low] probability that the
module region we choose is used up by vmalloc/vmap/ioremap
allocations, and so we need to be able to fall back to the general
vmalloc space.

>   2) and disables RANDOMIZE_MODULE_REGION_FULL with clang LTO.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 43ccddb7c8ec..a301fdc90319 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -997,8 +997,8 @@ config RELOCATABLE
>
>  config RANDOMIZE_BASE
>         bool "Randomize the address of the kernel image"
> -       select ARM64_MODULE_PLTS if MODULES
>         select RELOCATABLE
> +       select ARM64_MODULE_PLTS if RANDOMIZE_MODULE_REGION_FULL
>         help
>           Randomizes the virtual address at which the kernel image is
>           loaded, as a security feature that deters exploit attempts
> @@ -1016,7 +1016,7 @@ config RANDOMIZE_BASE
>
>  config RANDOMIZE_MODULE_REGION_FULL
>         bool "Randomize the module region independently from the core kernel"
> -       depends on RANDOMIZE_BASE
> +       depends on RANDOMIZE_BASE && !CLANG_LTO
>         default y
>         help
>           Randomizes the location of the module region without considering the
> --
> 2.15.0.403.gc27cc4dac6-goog
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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