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Message-ID: <nycvar.YSQ.7.76.1709041409530.8603@knanqh.ubzr>
Date: Mon, 4 Sep 2017 14:11:30 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc: linux-arm-kernel@...ts.infradead.org, kernel-hardening@...ts.openwall.com,
Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>,
Kees Cook <keescook@...omium.org>, Thomas Garnier <thgarnie@...gle.com>,
Marc Zyngier <marc.zyngier@....com>, Mark Rutland <mark.rutland@....com>,
Tony Lindgren <tony@...mide.com>, Matt Fleming <matt@...eblueprint.co.uk>,
Dave Martin <dave.martin@....com>
Subject: Re: [PATCH v2 19/29] ARM: kernel: make vmlinux buildable as a PIE
executable
On Sun, 3 Sep 2017, Ard Biesheuvel wrote:
> Update the build flags and linker script to allow vmlinux to be built
> as a PIE binary, which retains relocation information about absolute
> symbol references so that they can be fixed up at runtime. This will
> be used for implementing KASLR,
>
> Cc: Russell King <linux@...linux.org.uk>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
> arch/arm/Kconfig | 4 ++++
> arch/arm/Makefile | 5 +++++
> arch/arm/include/asm/assembler.h | 2 +-
> arch/arm/kernel/vmlinux.lds.S | 9 +++++++++
> include/linux/hidden.h | 20 ++++++++++++++++++++
> scripts/module-common.lds | 1 +
> 6 files changed, 40 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 61a0cb15067e..300add3b8023 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -2085,6 +2085,10 @@ config DMI
> firmware need to be enabled. This would require the DMI subsystem
> to be enabled much earlier than we do on ARM, which is non-trivial.
>
> +config RELOCATABLE
> + bool
> + select HAVE_ARCH_PREL32_RELOCATIONS
> +
You should probably make it depend on !XIP_KERNEL.
Other than that
Acked-by: Nicolas Pitre <nico@...aro.org>
>
> menu "CPU Power Management"
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index 47d3a1ab08d2..8544c8148b9f 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -52,6 +52,11 @@ AS += -EL
> LD += -EL
> endif
>
> +ifeq ($(CONFIG_RELOCATABLE),y)
> +KBUILD_CFLAGS += -fpic -include $(srctree)/include/linux/hidden.h
> +LDFLAGS_vmlinux += -pie -shared -Bsymbolic
> +endif
> +
> #
> # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and
> # later may result in code being generated that handles signed short and signed
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index ae43859a9000..0586e1c77a80 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -555,7 +555,7 @@ THUMB( orr \reg , \reg , #PSR_T_BIT )
> * mov_l - move a constant value or [relocated] address into a register
> */
> .macro mov_l, dst:req, imm:req
> - .if __LINUX_ARM_ARCH__ < 7
> + .if CONFIG_RELOCATABLE == 1 || __LINUX_ARM_ARCH__ < 7
> ldr \dst, =\imm
> .else
> movw \dst, #:lower16:\imm
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index c83a7ba737d6..5853d4be2067 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -89,6 +89,9 @@ SECTIONS
> #endif
> *(.discard)
> *(.discard.*)
> + *(.ARM.exidx.discard.text)
> + *(.interp .dynamic)
> + *(.dynsym .dynstr .hash)
> }
>
> . = PAGE_OFFSET + TEXT_OFFSET;
> @@ -209,6 +212,12 @@ SECTIONS
> __smpalt_end = .;
> }
> #endif
> + .rel.dyn : ALIGN(8) {
> + __rel_begin = .;
> + *(.rel .rel.* .rel.dyn)
> + }
> + __rel_end = ADDR(.rel.dyn) + SIZEOF(.rel.dyn);
> +
> .init.pv_table : {
> __pv_table_begin = .;
> *(.pv_table)
> diff --git a/include/linux/hidden.h b/include/linux/hidden.h
> new file mode 100644
> index 000000000000..781fb6f0c742
> --- /dev/null
> +++ b/include/linux/hidden.h
> @@ -0,0 +1,20 @@
> +/*
> + * GCC assumes that we are building shared libraries or hosted binaries
> + * when the -fpic or -fpie switches are used. This results in all references
> + * to symbols with external linkage to be redirected via entries in the global
> + * offset table (GOT), which keeps .text pages clean and reduces the footprint
> + * of CoWed dirty pages to the GOT itself. It also allows symbol preemption,
> + * which is mandatory under ELF rules for shared libraries.
> + *
> + * For the kernel, we use PIC so that we can relocate the executable image at
> + * runtime. This does not involve CoW or symbol preemption, and we'd rather
> + * have relative references instead of absolute ones whenever possible.
> + * So set the default visibility to hidden: this informs the compiler that
> + * none of our symbols will ever be exported from a shared library, allowing
> + * it to use relative references where possible.
> + *
> + * Note that simply passing -fvisibility=hidden is not sufficient to achieve
> + * this: In that case, definitions will be marked hidden, but declarations
> + * will not, and we still end up with GOT entries unnecessarily.
> + */
> +#pragma GCC visibility push(hidden)
> diff --git a/scripts/module-common.lds b/scripts/module-common.lds
> index d61b9e8678e8..55bb41f32c8a 100644
> --- a/scripts/module-common.lds
> +++ b/scripts/module-common.lds
> @@ -7,6 +7,7 @@ SECTIONS {
> /DISCARD/ : {
> *(.discard)
> *(.discard.*)
> + *(*.discard.*)
> }
>
> __ksymtab 0 : { *(SORT(___ksymtab+*)) }
> --
> 2.11.0
>
>
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