Commit a5e090ac authored by Russell King's avatar Russell King
Browse files

ARM: software-based priviledged-no-access support 



Provide a software-based implementation of the priviledged no access
support found in ARMv8.1.

Userspace pages are mapped using a different domain number from the
kernel and IO mappings.  If we switch the user domain to "no access"
when we enter the kernel, we can prevent the kernel from touching
userspace.

However, the kernel needs to be able to access userspace via the
various user accessor functions.  With the wrapping in the previous
patch, we can temporarily enable access when the kernel needs user
access, and re-disable it afterwards.

This allows us to trap non-intended accesses to userspace, eg, caused
by an inadvertent dereference of the LIST_POISON* values, which, with
appropriate user mappings setup, can be made to succeed.  This in turn
can allow use-after-free bugs to be further exploited than would
otherwise be possible.

Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 2190fed6

arch/arm/Kconfig

+15 −0
Original line number Diff line number Diff line
@@ -1694,6 +1694,21 @@ config HIGHPTE 
	bool "Allocate 2nd-level pagetables from highmem"

	depends on HIGHMEM



config CPU_SW_DOMAIN_PAN

	bool "Enable use of CPU domains to implement privileged no-access"

	depends on MMU && !ARM_LPAE

	default y

	help

	  Increase kernel security by ensuring that normal kernel accesses

	  are unable to access userspace addresses.  This can help prevent

	  use-after-free bugs becoming an exploitable privilege escalation

	  by ensuring that magic values (such as LIST_POISON) will always

	  fault when dereferenced.



	  CPUs with low-vector mappings use a best-efforts implementation.

	  Their lower 1MB needs to remain accessible for the vectors, but

	  the remainder of userspace will become appropriately inaccessible.



config HW_PERF_EVENTS

	bool "Enable hardware performance counter support for perf events"

	depends on PERF_EVENTS

arch/arm/include/asm/assembler.h

+30 −0
Original line number Diff line number Diff line
@@ -446,15 +446,45 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) 
	.endm



	.macro	uaccess_disable, tmp, isb=1

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	/*

	 * Whenever we re-enter userspace, the domains should always be

	 * set appropriately.

	 */

	mov	\tmp, #DACR_UACCESS_DISABLE

	mcr	p15, 0, \tmp, c3, c0, 0		@ Set domain register

	.if	\isb

	instr_sync

	.endif

#endif

	.endm



	.macro	uaccess_enable, tmp, isb=1

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	/*

	 * Whenever we re-enter userspace, the domains should always be

	 * set appropriately.

	 */

	mov	\tmp, #DACR_UACCESS_ENABLE

	mcr	p15, 0, \tmp, c3, c0, 0

	.if	\isb

	instr_sync

	.endif

#endif

	.endm



	.macro	uaccess_save, tmp

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	mrc	p15, 0, \tmp, c3, c0, 0

	str	\tmp, [sp, #S_FRAME_SIZE]

#endif

	.endm



	.macro	uaccess_restore

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	ldr	r0, [sp, #S_FRAME_SIZE]

	mcr	p15, 0, r0, c3, c0, 0

#endif

	.endm



	.macro	uaccess_save_and_disable, tmp

arch/arm/include/asm/domain.h

+19 −2
Original line number Diff line number Diff line
@@ -57,11 +57,29 @@ 
#define domain_mask(dom)	((3) << (2 * (dom)))

#define domain_val(dom,type)	((type) << (2 * (dom)))



#ifdef CONFIG_CPU_SW_DOMAIN_PAN

#define DACR_INIT \

	(domain_val(DOMAIN_USER, DOMAIN_NOACCESS) | \

	 domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \

	 domain_val(DOMAIN_IO, DOMAIN_CLIENT) | \

	 domain_val(DOMAIN_VECTORS, DOMAIN_CLIENT))

#else

#define DACR_INIT \

	(domain_val(DOMAIN_USER, DOMAIN_CLIENT) | \

	 domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \

	 domain_val(DOMAIN_IO, DOMAIN_CLIENT) | \

	 domain_val(DOMAIN_VECTORS, DOMAIN_CLIENT))

#endif



#define __DACR_DEFAULT \

	domain_val(DOMAIN_KERNEL, DOMAIN_CLIENT) | \

	domain_val(DOMAIN_IO, DOMAIN_CLIENT) | \

	domain_val(DOMAIN_VECTORS, DOMAIN_CLIENT)



#define DACR_UACCESS_DISABLE	\

	(__DACR_DEFAULT | domain_val(DOMAIN_USER, DOMAIN_NOACCESS))

#define DACR_UACCESS_ENABLE	\

	(__DACR_DEFAULT | domain_val(DOMAIN_USER, DOMAIN_CLIENT))



#ifndef __ASSEMBLY__
@@ -76,7 +94,6 @@ static inline unsigned int get_domain(void) 
	return domain;

}



#ifdef CONFIG_CPU_USE_DOMAINS

static inline void set_domain(unsigned val)

{

	asm volatile(
@@ -85,6 +102,7 @@ static inline void set_domain(unsigned val) 
	isb();

}



#ifdef CONFIG_CPU_USE_DOMAINS

#define modify_domain(dom,type)					\

	do {							\

		unsigned int domain = get_domain();		\
@@ -94,7 +112,6 @@ static inline void set_domain(unsigned val) 
	} while (0)



#else

static inline void set_domain(unsigned val) { }

static inline void modify_domain(unsigned dom, unsigned type)	{ }

#endif

arch/arm/include/asm/uaccess.h

+14 −0
Original line number Diff line number Diff line
@@ -57,11 +57,25 @@ extern int fixup_exception(struct pt_regs *regs); 
 */

static inline unsigned int uaccess_save_and_enable(void)

{

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	unsigned int old_domain = get_domain();



	/* Set the current domain access to permit user accesses */

	set_domain((old_domain & ~domain_mask(DOMAIN_USER)) |

		   domain_val(DOMAIN_USER, DOMAIN_CLIENT));



	return old_domain;

#else

	return 0;

#endif

}



static inline void uaccess_restore(unsigned int flags)

{

#ifdef CONFIG_CPU_SW_DOMAIN_PAN

	/* Restore the user access mask */

	set_domain(flags);

#endif

}



/*

arch/arm/kernel/process.c

+30 −6
Original line number Diff line number Diff line
@@ -129,12 +129,36 @@ void __show_regs(struct pt_regs *regs) 
	buf[4] = '\0';



#ifndef CONFIG_CPU_V7M

	{

		unsigned int domain = get_domain();

		const char *segment;



#ifdef CONFIG_CPU_SW_DOMAIN_PAN

		/*

		 * Get the domain register for the parent context. In user

		 * mode, we don't save the DACR, so lets use what it should

		 * be. For other modes, we place it after the pt_regs struct.

		 */

		if (user_mode(regs))

			domain = DACR_UACCESS_ENABLE;

		else

			domain = *(unsigned int *)(regs + 1);

#endif



		if ((domain & domain_mask(DOMAIN_USER)) ==

		    domain_val(DOMAIN_USER, DOMAIN_NOACCESS))

			segment = "none";

		else if (get_fs() == get_ds())

			segment = "kernel";

		else

			segment = "user";



		printk("Flags: %s  IRQs o%s  FIQs o%s  Mode %s  ISA %s  Segment %s\n",

			buf, interrupts_enabled(regs) ? "n" : "ff",

			fast_interrupts_enabled(regs) ? "n" : "ff",

			processor_modes[processor_mode(regs)],

		isa_modes[isa_mode(regs)],

		get_fs() == get_ds() ? "kernel" : "user");

			isa_modes[isa_mode(regs)], segment);

	}

#else

	printk("xPSR: %08lx\n", regs->ARM_cpsr);

#endif

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