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Date:   Wed, 29 May 2019 13:10:50 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Alexandre Ghiti <alex@ghiti.fr>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>,
        James Hogan <jhogan@kernel.org>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luis Chamberlain <mcgrof@kernel.org>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v4 05/14] arm64, mm: Make randomization selected by
 generic topdown mmap layout
Message-ID: <201905291310.E27265DACF@keescook>
References: <20190526134746.9315-1-alex@ghiti.fr>
 <20190526134746.9315-6-alex@ghiti.fr>
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On Sun, May 26, 2019 at 09:47:37AM -0400, Alexandre Ghiti wrote:
> This commits selects ARCH_HAS_ELF_RANDOMIZE when an arch uses the generic
> topdown mmap layout functions so that this security feature is on by
> default.
> Note that this commit also removes the possibility for arm64 to have elf
> randomization and no MMU: without MMU, the security added by randomization
> is worth nothing.
> 
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  arch/Kconfig                |  1 +
>  arch/arm64/Kconfig          |  1 -
>  arch/arm64/kernel/process.c |  8 --------
>  mm/util.c                   | 11 +++++++++--
>  4 files changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index df3ab04270fa..3732654446cc 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -710,6 +710,7 @@ config HAVE_ARCH_COMPAT_MMAP_BASES
>  config ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>  	bool
>  	depends on MMU
> +	select ARCH_HAS_ELF_RANDOMIZE
>  
>  config HAVE_COPY_THREAD_TLS
>  	bool
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 3d754c19c11e..403bd3fffdbc 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -15,7 +15,6 @@ config ARM64
>  	select ARCH_HAS_DMA_MMAP_PGPROT
>  	select ARCH_HAS_DMA_PREP_COHERENT
>  	select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI
> -	select ARCH_HAS_ELF_RANDOMIZE
>  	select ARCH_HAS_FAST_MULTIPLIER
>  	select ARCH_HAS_FORTIFY_SOURCE
>  	select ARCH_HAS_GCOV_PROFILE_ALL
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 3767fb21a5b8..3f85f8f2d665 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -535,14 +535,6 @@ unsigned long arch_align_stack(unsigned long sp)
>  	return sp & ~0xf;
>  }
>  
> -unsigned long arch_randomize_brk(struct mm_struct *mm)
> -{
> -	if (is_compat_task())
> -		return randomize_page(mm->brk, SZ_32M);
> -	else
> -		return randomize_page(mm->brk, SZ_1G);
> -}
> -
>  /*
>   * Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
>   */
> diff --git a/mm/util.c b/mm/util.c
> index 717f5d75c16e..8a38126edc74 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -319,7 +319,15 @@ unsigned long randomize_stack_top(unsigned long stack_top)
>  }
>  
>  #ifdef CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
> -#ifdef CONFIG_ARCH_HAS_ELF_RANDOMIZE
> +unsigned long arch_randomize_brk(struct mm_struct *mm)
> +{
> +	/* Is the current task 32bit ? */
> +	if (!IS_ENABLED(CONFIG_64BIT) || is_compat_task())
> +		return randomize_page(mm->brk, SZ_32M);
> +
> +	return randomize_page(mm->brk, SZ_1G);
> +}
> +
>  unsigned long arch_mmap_rnd(void)
>  {
>  	unsigned long rnd;
> @@ -333,7 +341,6 @@ unsigned long arch_mmap_rnd(void)
>  
>  	return rnd << PAGE_SHIFT;
>  }
> -#endif /* CONFIG_ARCH_HAS_ELF_RANDOMIZE */
>  
>  static int mmap_is_legacy(struct rlimit *rlim_stack)
>  {
> -- 
> 2.20.1
> 

-- 
Kees Cook