ARM: 8796/1: spectre-v1,v1.1: provide helpers for address sanitization

Introduce C and asm helpers to sanitize user address, taking the
address range they target into account.

Use asm helper for existing sanitization in __copy_from_user().
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/include/asm/assembler.h

@@ -467,6 +467,17 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 #endif
 	.endm
 
+	.macro uaccess_mask_range_ptr, addr:req, size:req, limit:req, tmp:req
+#ifdef CONFIG_CPU_SPECTRE
+	sub	\tmp, \limit, #1
+	subs	\tmp, \tmp, \addr	@ tmp = limit - 1 - addr
+	addhs	\tmp, \tmp, #1		@ if (tmp >= 0) {
+	subhss	\tmp, \tmp, \size	@ tmp = limit - (addr + size) }
+	movlo	\addr, #0		@ if (tmp < 0) addr = NULL
+	csdb
+#endif
+	.endm
+
 	.macro	uaccess_disable, tmp, isb=1
 #ifdef CONFIG_CPU_SW_DOMAIN_PAN
 	/*

arch/arm/include/asm/uaccess.h

@@ -99,6 +99,32 @@ static inline void set_fs(mm_segment_t fs)
 #define __inttype(x) \
 	__typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
 
+/*
+ * Sanitise a uaccess pointer such that it becomes NULL if addr+size
+ * is above the current addr_limit.
+ */
+#define uaccess_mask_range_ptr(ptr, size)			\
+	((__typeof__(ptr))__uaccess_mask_range_ptr(ptr, size))
+static inline void __user *__uaccess_mask_range_ptr(const void __user *ptr,
+						    size_t size)
+{
+	void __user *safe_ptr = (void __user *)ptr;
+	unsigned long tmp;
+
+	asm volatile(
+	"	sub	%1, %3, #1\n"
+	"	subs	%1, %1, %0\n"
+	"	addhs	%1, %1, #1\n"
+	"	subhss	%1, %1, %2\n"
+	"	movlo	%0, #0\n"
+	: "+r" (safe_ptr), "=&r" (tmp)
+	: "r" (size), "r" (current_thread_info()->addr_limit)
+	: "cc");
+
+	csdb();
+	return safe_ptr;
+}
+
 /*
  * Single-value transfer routines.  They automatically use the right
  * size if we just have the right pointer type.  Note that the functions

arch/arm/lib/copy_from_user.S

@@ -93,11 +93,7 @@ ENTRY(arm_copy_from_user)
 #ifdef CONFIG_CPU_SPECTRE
 	get_thread_info r3
 	ldr	r3, [r3, #TI_ADDR_LIMIT]
-	adds	ip, r1, r2	@ ip=addr+size
-	sub	r3, r3, #1	@ addr_limit - 1
-	cmpcc	ip, r3		@ if (addr+size > addr_limit - 1)
-	movcs	r1, #0		@ addr = NULL
-	csdb
+	uaccess_mask_range_ptr r1, r2, r3, ip
 #endif
 
 #include "copy_template.S"