arm64: cpufeatures: Allow disabling of Pointer Auth from the command-line

In order to be able to disable Pointer Authentication  at runtime,
whether it is for testing purposes, or to work around HW issues,
let's add support for overriding the ID_AA64ISAR1_EL1.{GPI,GPA,API,APA}
fields.

This is further mapped on the arm64.nopauth command-line alias.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: David Brazdil <dbrazdil@google.com>
Tested-by: Srinivas Ramana <sramana@codeaurora.org>
Link: https://lore.kernel.org/r/20210208095732.3267263-23-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -376,6 +376,9 @@
 	arm64.nobti	[ARM64] Unconditionally disable Branch Target
 			Identification support
 
+	arm64.nopauth	[ARM64] Unconditionally disable Pointer Authentication
+			support
+
 	ataflop=	[HW,M68k]
 
 	atarimouse=	[HW,MOUSE] Atari Mouse

arch/arm64/include/asm/cpufeature.h

@@ -820,6 +820,7 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)
 
 extern struct arm64_ftr_override id_aa64mmfr1_override;
 extern struct arm64_ftr_override id_aa64pfr1_override;
+extern struct arm64_ftr_override id_aa64isar1_override;
 
 u32 get_kvm_ipa_limit(void);
 void dump_cpu_features(void);

arch/arm64/kernel/cpufeature.c

@@ -559,6 +559,7 @@ static const struct arm64_ftr_bits ftr_raz[] = {
 
 struct arm64_ftr_override __ro_after_init id_aa64mmfr1_override;
 struct arm64_ftr_override __ro_after_init id_aa64pfr1_override;
+struct arm64_ftr_override __ro_after_init id_aa64isar1_override;
 
 static const struct __ftr_reg_entry {
 	u32			sys_id;
@@ -604,7 +605,8 @@ static const struct __ftr_reg_entry {
 
 	/* Op1 = 0, CRn = 0, CRm = 6 */
 	ARM64_FTR_REG(SYS_ID_AA64ISAR0_EL1, ftr_id_aa64isar0),
-	ARM64_FTR_REG(SYS_ID_AA64ISAR1_EL1, ftr_id_aa64isar1),
+	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64ISAR1_EL1, ftr_id_aa64isar1,
+			       &id_aa64isar1_override),
 
 	/* Op1 = 0, CRn = 0, CRm = 7 */
 	ARM64_FTR_REG(SYS_ID_AA64MMFR0_EL1, ftr_id_aa64mmfr0),

arch/arm64/kernel/idreg-override.c

@@ -46,6 +46,18 @@ static const struct ftr_set_desc pfr1 __initconst = {
 	},
 };
 
+static const struct ftr_set_desc isar1 __initconst = {
+	.name		= "id_aa64isar1",
+	.override	= &id_aa64isar1_override,
+	.fields		= {
+	        { "gpi", ID_AA64ISAR1_GPI_SHIFT },
+	        { "gpa", ID_AA64ISAR1_GPA_SHIFT },
+	        { "api", ID_AA64ISAR1_API_SHIFT },
+	        { "apa", ID_AA64ISAR1_APA_SHIFT },
+		{}
+	},
+};
+
 extern struct arm64_ftr_override kaslr_feature_override;
 
 static const struct ftr_set_desc kaslr __initconst = {
@@ -62,6 +74,7 @@ static const struct ftr_set_desc kaslr __initconst = {
 static const struct ftr_set_desc * const regs[] __initconst = {
 	&mmfr1,
 	&pfr1,
+	&isar1,
 	&kaslr,
 };
 
@@ -72,6 +85,9 @@ static const struct {
 	{ "kvm-arm.mode=nvhe",		"id_aa64mmfr1.vh=0" },
 	{ "kvm-arm.mode=protected",	"id_aa64mmfr1.vh=0" },
 	{ "arm64.nobti",		"id_aa64pfr1.bt=0" },
+	{ "arm64.nopauth",
+	  "id_aa64isar1.gpi=0 id_aa64isar1.gpa=0 "
+	  "id_aa64isar1.api=0 id_aa64isar1.apa=0"	   },
 	{ "nokaslr",			"kaslr.disabled=1" },
 };