arm64: Kconfig: Re-jig CONFIG options for 52-bit VA

Enabling 52-bit VAs for userspace is pretty confusing, since it requires
you to select "48-bit" virtual addressing in the Kconfig.

Rework the logic so that 52-bit user virtual addressing is advertised in
the "Virtual address space size" choice, along with some help text to
describe its interaction with Pointer Authentication. The EXPERT-only
option to force all user mappings to the 52-bit range is then made
available immediately below the VA size selection.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -682,15 +682,43 @@ config ARM64_VA_BITS_47
 config ARM64_VA_BITS_48
 	bool "48-bit"
 
+config ARM64_USER_VA_BITS_52
+	bool "52-bit (user)"
+	depends on ARM64_64K_PAGES && (ARM64_PAN || !ARM64_SW_TTBR0_PAN)
+	help
+	  Enable 52-bit virtual addressing for userspace when explicitly
+	  requested via a hint to mmap(). The kernel will continue to
+	  use 48-bit virtual addresses for its own mappings.
+
+	  NOTE: Enabling 52-bit virtual addressing in conjunction with
+	  ARMv8.3 Pointer Authentication will result in the PAC being
+	  reduced from 7 bits to 3 bits, which may have a significant
+	  impact on its susceptibility to brute-force attacks.
+
+	  If unsure, select 48-bit virtual addressing instead.
+
 endchoice
 
+config ARM64_FORCE_52BIT
+	bool "Force 52-bit virtual addresses for userspace"
+	depends on ARM64_USER_VA_BITS_52 && EXPERT
+	help
+	  For systems with 52-bit userspace VAs enabled, the kernel will attempt
+	  to maintain compatibility with older software by providing 48-bit VAs
+	  unless a hint is supplied to mmap.
+
+	  This configuration option disables the 48-bit compatibility logic, and
+	  forces all userspace addresses to be 52-bit on HW that supports it. One
+	  should only enable this configuration option for stress testing userspace
+	  memory management code. If unsure say N here.
+
 config ARM64_VA_BITS
 	int
 	default 36 if ARM64_VA_BITS_36
 	default 39 if ARM64_VA_BITS_39
 	default 42 if ARM64_VA_BITS_42
 	default 47 if ARM64_VA_BITS_47
-	default 48 if ARM64_VA_BITS_48
+	default 48 if ARM64_VA_BITS_48 || ARM64_USER_VA_BITS_52
 
 choice
 	prompt "Physical address space size"
@@ -716,10 +744,6 @@ config ARM64_PA_BITS_52
 
 endchoice
 
-config ARM64_52BIT_VA
-	def_bool y
-	depends on ARM64_VA_BITS_48 && ARM64_64K_PAGES && (ARM64_PAN || !ARM64_SW_TTBR0_PAN)
-
 config ARM64_PA_BITS
 	int
 	default 48 if ARM64_PA_BITS_48
@@ -1186,19 +1210,6 @@ config ARM64_CNP
 	  at runtime, and does not affect PEs that do not implement
 	  this feature.
 
-config ARM64_FORCE_52BIT
-	bool "Force 52-bit virtual addresses for userspace"
-	depends on ARM64_52BIT_VA && EXPERT
-	help
-	  For systems with 52-bit userspace VAs enabled, the kernel will attempt
-	  to maintain compatibility with older software by providing 48-bit VAs
-	  unless a hint is supplied to mmap.
-
-	  This configuration option disables the 48-bit compatibility logic, and
-	  forces all userspace addresses to be 52-bit on HW that supports it. One
-	  should only enable this configuration option for stress testing userspace
-	  memory management code. If unsure say N here.
-
 endmenu
 
 config ARM64_SVE

arch/arm64/include/asm/assembler.h

@@ -549,7 +549,7 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
  * 	ttbr: Value of ttbr to set, modified.
  */
 	.macro	offset_ttbr1, ttbr
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 	orr	\ttbr, \ttbr, #TTBR1_BADDR_4852_OFFSET
 #endif
 	.endm
@@ -560,7 +560,7 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
  * to be nop'ed out when dealing with 52-bit kernel VAs.
  */
 	.macro	restore_ttbr1, ttbr
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 	bic	\ttbr, \ttbr, #TTBR1_BADDR_4852_OFFSET
 #endif
 	.endm

arch/arm64/include/asm/mmu_context.h

@@ -74,7 +74,7 @@ extern u64 idmap_ptrs_per_pgd;
 
 static inline bool __cpu_uses_extended_idmap(void)
 {
-	if (IS_ENABLED(CONFIG_ARM64_52BIT_VA))
+	if (IS_ENABLED(CONFIG_ARM64_USER_VA_BITS_52))
 		return false;
 
 	return unlikely(idmap_t0sz != TCR_T0SZ(VA_BITS));

arch/arm64/include/asm/pgtable-hwdef.h

@@ -80,7 +80,7 @@
 #define PGDIR_SHIFT		ARM64_HW_PGTABLE_LEVEL_SHIFT(4 - CONFIG_PGTABLE_LEVELS)
 #define PGDIR_SIZE		(_AC(1, UL) << PGDIR_SHIFT)
 #define PGDIR_MASK		(~(PGDIR_SIZE-1))
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 #define PTRS_PER_PGD		(1 << (52 - PGDIR_SHIFT))
 #else
 #define PTRS_PER_PGD		(1 << (VA_BITS - PGDIR_SHIFT))
@@ -310,7 +310,7 @@
 #define TTBR_BADDR_MASK_52	(((UL(1) << 46) - 1) << 2)
 #endif
 
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 /* Must be at least 64-byte aligned to prevent corruption of the TTBR */
 #define TTBR1_BADDR_4852_OFFSET	(((UL(1) << (52 - PGDIR_SHIFT)) - \
 				 (UL(1) << (48 - PGDIR_SHIFT))) * 8)

arch/arm64/include/asm/processor.h

@@ -20,11 +20,11 @@
 #define __ASM_PROCESSOR_H
 
 #define KERNEL_DS		UL(-1)
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 #define USER_DS			((UL(1) << 52) - 1)
 #else
 #define USER_DS			((UL(1) << VA_BITS) - 1)
-#endif /* CONFIG_ARM64_52BIT_VA */
+#endif /* CONFIG_ARM64_USER_VA_BITS_52 */
 
 /*
  * On arm64 systems, unaligned accesses by the CPU are cheap, and so there is

arch/arm64/kernel/head.S

@@ -318,7 +318,7 @@ __create_page_tables:
 	adrp	x0, idmap_pg_dir
 	adrp	x3, __idmap_text_start		// __pa(__idmap_text_start)
 
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 	mrs_s	x6, SYS_ID_AA64MMFR2_EL1
 	and	x6, x6, #(0xf << ID_AA64MMFR2_LVA_SHIFT)
 	mov	x5, #52
@@ -800,7 +800,7 @@ ENTRY(__enable_mmu)
 ENDPROC(__enable_mmu)
 
 ENTRY(__cpu_secondary_check52bitva)
-#ifdef CONFIG_ARM64_52BIT_VA
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
 	ldr_l	x0, vabits_user
 	cmp	x0, #52
 	b.ne	2f

arch/arm64/kernel/smp.c

@@ -139,7 +139,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 		if (!cpu_online(cpu)) {
 			pr_crit("CPU%u: failed to come online\n", cpu);
 
-			if (IS_ENABLED(CONFIG_ARM64_52BIT_VA) && va52mismatch)
+			if (IS_ENABLED(CONFIG_ARM64_USER_VA_BITS_52) && va52mismatch)
 				pr_crit("CPU%u: does not support 52-bit VAs\n", cpu);
 
 			ret = -EIO;

arch/arm64/mm/proc.S

@@ -451,8 +451,8 @@ ENTRY(__cpu_setup)
 			TCR_TG_FLAGS | TCR_KASLR_FLAGS | TCR_ASID16 | \
 			TCR_TBI0 | TCR_A1
 
-#ifdef CONFIG_ARM64_52BIT_VA
-	ldr_l 		x9, vabits_user
+#ifdef CONFIG_ARM64_USER_VA_BITS_52
+	ldr_l		x9, vabits_user
 	sub		x9, xzr, x9
 	add		x9, x9, #64
 #else