arm64: cpufeature: Detect SSBS and advertise to userspace

Armv8.5 introduces a new PSTATE bit known as Speculative Store Bypass
Safe (SSBS) which can be used as a mitigation against Spectre variant 4.

Additionally, a CPU may provide instructions to manipulate PSTATE.SSBS
directly, so that userspace can toggle the SSBS control without trapping
to the kernel.

This patch probes for the existence of SSBS and advertise the new instructions
to userspace if they exist.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cpucaps.h

@@ -52,7 +52,8 @@
 #define ARM64_MISMATCHED_CACHE_TYPE		31
 #define ARM64_HAS_STAGE2_FWB			32
 #define ARM64_HAS_CRC32				33
+#define ARM64_SSBS				34
 
-#define ARM64_NCAPS				34
+#define ARM64_NCAPS				35
 
 #endif /* __ASM_CPUCAPS_H */

arch/arm64/include/asm/sysreg.h

@@ -419,6 +419,7 @@
 #define SYS_ICH_LR15_EL2		__SYS__LR8_EL2(7)
 
 /* Common SCTLR_ELx flags. */
+#define SCTLR_ELx_DSSBS	(1UL << 44)
 #define SCTLR_ELx_EE    (1 << 25)
 #define SCTLR_ELx_IESB	(1 << 21)
 #define SCTLR_ELx_WXN	(1 << 19)
@@ -439,7 +440,7 @@
 			 (1 << 10) | (1 << 13) | (1 << 14) | (1 << 15) | \
 			 (1 << 17) | (1 << 20) | (1 << 24) | (1 << 26) | \
 			 (1 << 27) | (1 << 30) | (1 << 31) | \
-			 (0xffffffffUL << 32))
+			 (0xffffefffUL << 32))
 
 #ifdef CONFIG_CPU_BIG_ENDIAN
 #define ENDIAN_SET_EL2		SCTLR_ELx_EE
@@ -453,7 +454,7 @@
 #define SCTLR_EL2_SET	(SCTLR_ELx_IESB   | ENDIAN_SET_EL2   | SCTLR_EL2_RES1)
 #define SCTLR_EL2_CLEAR	(SCTLR_ELx_M      | SCTLR_ELx_A    | SCTLR_ELx_C   | \
 			 SCTLR_ELx_SA     | SCTLR_ELx_I    | SCTLR_ELx_WXN | \
-			 ENDIAN_CLEAR_EL2 | SCTLR_EL2_RES0)
+			 SCTLR_ELx_DSSBS | ENDIAN_CLEAR_EL2 | SCTLR_EL2_RES0)
 
 #if (SCTLR_EL2_SET ^ SCTLR_EL2_CLEAR) != 0xffffffffffffffff
 #error "Inconsistent SCTLR_EL2 set/clear bits"
@@ -477,7 +478,7 @@
 			 (1 << 29))
 #define SCTLR_EL1_RES0  ((1 << 6)  | (1 << 10) | (1 << 13) | (1 << 17) | \
 			 (1 << 27) | (1 << 30) | (1 << 31) | \
-			 (0xffffffffUL << 32))
+			 (0xffffefffUL << 32))
 
 #ifdef CONFIG_CPU_BIG_ENDIAN
 #define ENDIAN_SET_EL1		(SCTLR_EL1_E0E | SCTLR_ELx_EE)
@@ -494,7 +495,7 @@
 			 ENDIAN_SET_EL1 | SCTLR_EL1_UCI  | SCTLR_EL1_RES1)
 #define SCTLR_EL1_CLEAR	(SCTLR_ELx_A   | SCTLR_EL1_CP15BEN | SCTLR_EL1_ITD    |\
 			 SCTLR_EL1_UMA | SCTLR_ELx_WXN     | ENDIAN_CLEAR_EL1 |\
-			 SCTLR_EL1_RES0)
+			 SCTLR_ELx_DSSBS | SCTLR_EL1_RES0)
 
 #if (SCTLR_EL1_SET ^ SCTLR_EL1_CLEAR) != 0xffffffffffffffff
 #error "Inconsistent SCTLR_EL1 set/clear bits"
@@ -544,6 +545,13 @@
 #define ID_AA64PFR0_EL0_64BIT_ONLY	0x1
 #define ID_AA64PFR0_EL0_32BIT_64BIT	0x2
 
+/* id_aa64pfr1 */
+#define ID_AA64PFR1_SSBS_SHIFT		4
+
+#define ID_AA64PFR1_SSBS_PSTATE_NI	0
+#define ID_AA64PFR1_SSBS_PSTATE_ONLY	1
+#define ID_AA64PFR1_SSBS_PSTATE_INSNS	2
+
 /* id_aa64mmfr0 */
 #define ID_AA64MMFR0_TGRAN4_SHIFT	28
 #define ID_AA64MMFR0_TGRAN64_SHIFT	24

arch/arm64/include/uapi/asm/hwcap.h

@@ -48,5 +48,6 @@
 #define HWCAP_USCAT		(1 << 25)
 #define HWCAP_ILRCPC		(1 << 26)
 #define HWCAP_FLAGM		(1 << 27)
+#define HWCAP_SSBS		(1 << 28)
 
 #endif /* _UAPI__ASM_HWCAP_H */

arch/arm64/kernel/cpufeature.c

@@ -164,6 +164,11 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
 	ARM64_FTR_END,
 };
 
+static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
+	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_SSBS_SHIFT, 4, ID_AA64PFR1_SSBS_PSTATE_NI),
+	ARM64_FTR_END,
+};
+
 static const struct arm64_ftr_bits ftr_id_aa64mmfr0[] = {
 	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_TGRAN4_SHIFT, 4, ID_AA64MMFR0_TGRAN4_NI),
 	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_TGRAN64_SHIFT, 4, ID_AA64MMFR0_TGRAN64_NI),
@@ -371,7 +376,7 @@ static const struct __ftr_reg_entry {
 
 	/* Op1 = 0, CRn = 0, CRm = 4 */
 	ARM64_FTR_REG(SYS_ID_AA64PFR0_EL1, ftr_id_aa64pfr0),
-	ARM64_FTR_REG(SYS_ID_AA64PFR1_EL1, ftr_raz),
+	ARM64_FTR_REG(SYS_ID_AA64PFR1_EL1, ftr_id_aa64pfr1),
 	ARM64_FTR_REG(SYS_ID_AA64ZFR0_EL1, ftr_raz),
 
 	/* Op1 = 0, CRn = 0, CRm = 5 */
@@ -657,7 +662,6 @@ void update_cpu_features(int cpu,
 
 	/*
 	 * EL3 is not our concern.
-	 * ID_AA64PFR1 is currently RES0.
 	 */
 	taint |= check_update_ftr_reg(SYS_ID_AA64PFR0_EL1, cpu,
 				      info->reg_id_aa64pfr0, boot->reg_id_aa64pfr0);
@@ -1231,6 +1235,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.field_pos = ID_AA64ISAR0_CRC32_SHIFT,
 		.min_field_value = 1,
 	},
+	{
+		.desc = "Speculative Store Bypassing Safe (SSBS)",
+		.capability = ARM64_SSBS,
+		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
+		.matches = has_cpuid_feature,
+		.sys_reg = SYS_ID_AA64PFR1_EL1,
+		.field_pos = ID_AA64PFR1_SSBS_SHIFT,
+		.sign = FTR_UNSIGNED,
+		.min_field_value = ID_AA64PFR1_SSBS_PSTATE_ONLY,
+	},
 	{},
 };
 
@@ -1276,6 +1290,7 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 #ifdef CONFIG_ARM64_SVE
 	HWCAP_CAP(SYS_ID_AA64PFR0_EL1, ID_AA64PFR0_SVE_SHIFT, FTR_UNSIGNED, ID_AA64PFR0_SVE, CAP_HWCAP, HWCAP_SVE),
 #endif
+	HWCAP_CAP(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_SSBS_SHIFT, FTR_UNSIGNED, ID_AA64PFR1_SSBS_PSTATE_INSNS, CAP_HWCAP, HWCAP_SSBS),
 	{},
 };
 

arch/arm64/kernel/cpuinfo.c

@@ -81,6 +81,7 @@ static const char *const hwcap_str[] = {
 	"uscat",
 	"ilrcpc",
 	"flagm",
+	"ssbs",
 	NULL
 };