Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	James Hogan <james.hogan@imgtec.com>,
	Pratyush Anand <panand@redhat.com>,
	Ingo Molnar <mingo@kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	linux-s390@vger.kernel.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	kernel-hardening@lists.openwall.com
Subject: Re: [kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE
Date: Fri, 23 Jun 2017 06:57:14 +0000	[thread overview]
Message-ID: <CAKv+Gu_2Wg_D+4c3-iqMr9hbKkGVdBtW6Bmxj-4=FOG9hVN5vA@mail.gmail.com> (raw)
In-Reply-To: <1498154792-49952-3-git-send-email-keescook@chromium.org>

Hi Kees,

On 22 June 2017 at 18:06, Kees Cook <keescook@chromium.org> wrote:
> Now that explicitly executed loaders are loaded in the mmap region,
> position PIE binaries lower in the address space to avoid possible
> collisions with mmap or stack regions. For 64-bit, align to 4GB to
> allow runtimes to use the entire 32-bit address space for 32-bit
> pointers.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm64/include/asm/elf.h | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 5d1700425efe..f742af8f7c42 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -113,12 +113,13 @@
>  #define ELF_EXEC_PAGESIZE      PAGE_SIZE
>
>  /*
> - * This is the location that an ET_DYN program is loaded if exec'ed.  Typical
> - * use of this is to invoke "./ld.so someprog" to test out a new version of
> - * the loader.  We need to make sure that it is out of the way of the program
> - * that it will "exec", and that there is sufficient room for the brk.
> + * This is the base location for PIE (ET_DYN with INTERP) loads. On
> + * 64-bit, this is raised to 4GB to leave the entire 32-bit address
> + * space open for things that want to use the area for 32-bit pointers.
>   */
> -#define ELF_ET_DYN_BASE        (2 * TASK_SIZE_64 / 3)
> +#define ELF_ET_DYN_BASE                (test_thread_flag(TIF_32BIT) ?  \
> +                                       0x000400000UL :         \
> +                                       0x100000000UL)
>

Why are you merging this with the COMPAT definition?

>  #ifndef __ASSEMBLY__
>
> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
>
>  #ifdef CONFIG_COMPAT
>
> -#define COMPAT_ELF_ET_DYN_BASE         (2 * TASK_SIZE_32 / 3)
> -
>  /* AArch32 registers. */
>  #define COMPAT_ELF_NGREG               18
>  typedef unsigned int                   compat_elf_greg_t;
> --
> 2.7.4
>