Commit af1b3cf2 authored by Vincenzo Frascino's avatar Vincenzo Frascino Committed by Will Deacon
Browse files

arm64: compat: Add KUSER_HELPERS config option 



When kuser helpers are enabled the kernel maps the relative code at
a fixed address (0xffff0000). Making configurable the option to disable
them means that the kernel can remove this mapping and any access to
this memory area results in a sigfault.

Add a KUSER_HELPERS config option that can be used to disable the
mapping when it is turned off.

This option can be turned off if and only if the applications are
designed specifically for the platform and they do not make use of the
kuser helpers code.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: default avatarVincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
[will: Use IS_ENABLED() instead of #ifdef]
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent 1255a734

arch/arm64/Kconfig

+28 −0
Original line number Diff line number Diff line
@@ -1498,6 +1498,34 @@ config COMPAT 


	  If you want to execute 32-bit userspace applications, say Y.



config KUSER_HELPERS

	bool "Enable kuser helpers page for 32 bit applications."

	depends on COMPAT

	default y

	help

	  Warning: disabling this option may break 32-bit user programs.



	  Provide kuser helpers to compat tasks. The kernel provides

	  helper code to userspace in read only form at a fixed location

	  to allow userspace to be independent of the CPU type fitted to

	  the system. This permits binaries to be run on ARMv4 through

	  to ARMv8 without modification.



	  See Documentation/arm/kernel_user_helpers.txt for details.



	  However, the fixed address nature of these helpers can be used

	  by ROP (return orientated programming) authors when creating

	  exploits.



	  If all of the binaries and libraries which run on your platform

	  are built specifically for your platform, and make no use of

	  these helpers, then you can turn this option off to hinder

	  such exploits. However, in that case, if a binary or library

	  relying on those helpers is run, it will not function correctly.



	  Say N here only if you are absolutely certain that you do not

	  need these helpers; otherwise, the safe option is to say Y.



config SYSVIPC_COMPAT

	def_bool y

	depends on COMPAT && SYSVIPC

arch/arm64/kernel/Makefile

+2 −1
Original line number Diff line number Diff line
@@ -27,8 +27,9 @@ OBJCOPYFLAGS := --prefix-symbols=__efistub_ 
$(obj)/%.stub.o: $(obj)/%.o FORCE

	$(call if_changed,objcopy)



obj-$(CONFIG_COMPAT)			+= sys32.o kuser32.o signal32.o 	\

obj-$(CONFIG_COMPAT)			+= sys32.o signal32.o			\

					   sigreturn32.o sys_compat.o

obj-$(CONFIG_KUSER_HELPERS)		+= kuser32.o

obj-$(CONFIG_FUNCTION_TRACER)		+= ftrace.o entry-ftrace.o

obj-$(CONFIG_MODULES)			+= module.o

obj-$(CONFIG_ARM64_MODULE_PLTS)		+= module-plts.o

arch/arm64/kernel/kuser32.S

+3 −4
Original line number Diff line number Diff line
@@ -6,10 +6,9 @@ 
 * Copyright (C) 2005-2011 Nicolas Pitre <nico@fluxnic.net>

 * Copyright (C) 2012-2018 ARM Ltd.

 *

 * Each segment is 32-byte aligned and will be moved to the top of the high

 * vector page.  New segments (if ever needed) must be added in front of

 * existing ones.  This mechanism should be used only for things that are

 * really small and justified, and not be abused freely.

 * The kuser helpers below are mapped at a fixed address by

 * aarch32_setup_additional_pages() and are provided for compatibility

 * reasons with 32 bit (aarch32) applications that need them.

 *

 * See Documentation/arm/kernel_user_helpers.txt for formal definitions.

 */

arch/arm64/kernel/vdso.c

+6 −0
Original line number Diff line number Diff line
@@ -74,6 +74,9 @@ static int aarch32_alloc_kuser_vdso_page(void) 
	int kuser_sz = __kuser_helper_end - __kuser_helper_start;

	unsigned long vdso_page;



	if (!IS_ENABLED(CONFIG_KUSER_HELPERS))

		return 0;



	vdso_page = get_zeroed_page(GFP_ATOMIC);

	if (!vdso_page)

		return -ENOMEM;
@@ -112,6 +115,9 @@ static int aarch32_kuser_helpers_setup(struct mm_struct *mm) 
{

	void *ret;



	if (!IS_ENABLED(CONFIG_KUSER_HELPERS))

		return 0;



	/*

	 * Avoid VM_MAYWRITE for compatibility with arch/arm/, where it's

	 * not safe to CoW the page containing the CPU exception vectors.

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