arm64: Factor out PAN enabling/disabling into separate uaccess_* macros

This patch moves the directly coded alternatives for turning PAN on/off
into separate uaccess_{enable,disable} macros or functions. The asm
macros take a few arguments which will be used in subsequent patches.

Note that any (unlikely) access that the compiler might generate between
uaccess_enable() and uaccess_disable(), other than those explicitly
specified by the user access code, will not be protected by PAN.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/armv8_deprecated.c

@@ -14,7 +14,6 @@
 #include <linux/slab.h>
 #include <linux/sysctl.h>
 
-#include <asm/alternative.h>
 #include <asm/cpufeature.h>
 #include <asm/insn.h>
 #include <asm/opcodes.h>
@@ -285,10 +284,10 @@ static void __init register_insn_emulation_sysctl(struct ctl_table *table)
 #define __SWP_LL_SC_LOOPS	4
 
 #define __user_swpX_asm(data, addr, res, temp, temp2, B)	\
+do {								\
+	uaccess_enable();					\
 	__asm__ __volatile__(					\
 	"	mov		%w3, %w7\n"			\
-	ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,	\
-		    CONFIG_ARM64_PAN)				\
 	"0:	ldxr"B"		%w2, [%4]\n"			\
 	"1:	stxr"B"		%w0, %w1, [%4]\n"		\
 	"	cbz		%w0, 2f\n"			\
@@ -306,12 +305,12 @@ static void __init register_insn_emulation_sysctl(struct ctl_table *table)
 	"	.popsection"					\
 	_ASM_EXTABLE(0b, 4b)					\
 	_ASM_EXTABLE(1b, 4b)					\
-	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
-		CONFIG_ARM64_PAN)				\
 	: "=&r" (res), "+r" (data), "=&r" (temp), "=&r" (temp2)	\
 	: "r" (addr), "i" (-EAGAIN), "i" (-EFAULT),		\
 	  "i" (__SWP_LL_SC_LOOPS)				\
-	: "memory")
+	: "memory");						\
+	uaccess_disable();					\
+} while (0)
 
 #define __user_swp_asm(data, addr, res, temp, temp2) \
 	__user_swpX_asm(data, addr, res, temp, temp2, "")