arm64: Use the clearbhb instruction in mitigations

commit 228a26b912287934789023b4132ba76065d9491c upstream.

Future CPUs may implement a clearbhb instruction that is sufficient
to mitigate SpectreBHB. CPUs that implement this instruction, but
not CSV2.3 must be affected by Spectre-BHB.

Add support to use this instruction as the BHB mitigation on CPUs
that support it. The instruction is in the hint space, so it will
be treated by a NOP as older CPUs.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[ modified for stable: Use a KVM vector template instead of alternatives,
  removed bitmap of mitigations ]
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/include/asm/assembler.h

@@ -97,6 +97,13 @@
 	hint	#20
 	.endm
 
+/*
+ * Clear Branch History instruction
+ */
+	.macro clearbhb
+	hint	#22
+	.endm
+
 /*
  * Speculation barrier
  */

arch/arm64/include/asm/cpufeature.h

@@ -621,6 +621,19 @@ static inline bool supports_csv2p3(int scope)
 	return csv2_val == 3;
 }
 
+static inline bool supports_clearbhb(int scope)
+{
+	u64 isar2;
+
+	if (scope == SCOPE_LOCAL_CPU)
+		isar2 = read_sysreg_s(SYS_ID_AA64ISAR2_EL1);
+	else
+		isar2 = read_sanitised_ftr_reg(SYS_ID_AA64ISAR2_EL1);
+
+	return cpuid_feature_extract_unsigned_field(isar2,
+						    ID_AA64ISAR2_CLEARBHB_SHIFT);
+}
+
 static inline bool system_supports_32bit_el0(void)
 {
 	return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);

arch/arm64/include/asm/insn.h

@@ -65,6 +65,7 @@ enum aarch64_insn_hint_cr_op {
 	AARCH64_INSN_HINT_PSB  = 0x11 << 5,
 	AARCH64_INSN_HINT_TSB  = 0x12 << 5,
 	AARCH64_INSN_HINT_CSDB = 0x14 << 5,
+	AARCH64_INSN_HINT_CLEARBHB = 0x16 << 5,
 
 	AARCH64_INSN_HINT_BTI   = 0x20 << 5,
 	AARCH64_INSN_HINT_BTIC  = 0x22 << 5,

arch/arm64/include/asm/kvm_asm.h

@@ -37,6 +37,7 @@
 #define __SMCCC_WORKAROUND_1_SMC_SZ 36
 #define __SMCCC_WORKAROUND_3_SMC_SZ 36
 #define __SPECTRE_BHB_LOOP_SZ       44
+#define __SPECTRE_BHB_CLEARBHB_SZ   12
 
 #define KVM_HOST_SMCCC_ID(id)						\
 	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
@@ -205,6 +206,7 @@ extern char __smccc_workaround_3_smc[__SMCCC_WORKAROUND_3_SMC_SZ];
 extern char __spectre_bhb_loop_k8[__SPECTRE_BHB_LOOP_SZ];
 extern char __spectre_bhb_loop_k24[__SPECTRE_BHB_LOOP_SZ];
 extern char __spectre_bhb_loop_k32[__SPECTRE_BHB_LOOP_SZ];
+extern char __spectre_bhb_clearbhb[__SPECTRE_BHB_LOOP_SZ];
 
 /*
  * Obtain the PC-relative address of a kernel symbol

arch/arm64/include/asm/sysreg.h

@@ -689,6 +689,7 @@
 #define ID_AA64ISAR1_GPI_IMP_DEF		0x1
 
 /* id_aa64isar2 */
+#define ID_AA64ISAR2_CLEARBHB_SHIFT	28
 #define ID_AA64ISAR2_RPRES_SHIFT	4
 #define ID_AA64ISAR2_WFXT_SHIFT		0
 

arch/arm64/include/asm/vectors.h

@@ -32,6 +32,12 @@ enum arm64_bp_harden_el1_vectors {
 	 * canonical vectors.
 	 */
 	EL1_VECTOR_BHB_FW,
+
+	/*
+	 * Use the ClearBHB instruction, before branching to the canonical
+	 * vectors.
+	 */
+	EL1_VECTOR_BHB_CLEAR_INSN,
 #endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
 
 	/*
@@ -43,6 +49,7 @@ enum arm64_bp_harden_el1_vectors {
 #ifndef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
 #define EL1_VECTOR_BHB_LOOP		-1
 #define EL1_VECTOR_BHB_FW		-1
+#define EL1_VECTOR_BHB_CLEAR_INSN	-1
 #endif /* !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
 
 /* The vectors to use on return from EL0. e.g. to remap the kernel */

arch/arm64/kernel/cpufeature.c

@@ -211,6 +211,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64isar2[] = {
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_HIGHER_SAFE, ID_AA64ISAR2_CLEARBHB_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR2_RPRES_SHIFT, 4, 0),
 	ARM64_FTR_END,
 };

arch/arm64/kernel/entry.S

@@ -827,6 +827,7 @@ alternative_else_nop_endif
 #define BHB_MITIGATION_NONE	0
 #define BHB_MITIGATION_LOOP	1
 #define BHB_MITIGATION_FW	2
+#define BHB_MITIGATION_INSN	3
 
 	.macro tramp_ventry, vector_start, regsize, kpti, bhb
 	.align	7
@@ -843,6 +844,11 @@ alternative_else_nop_endif
 	__mitigate_spectre_bhb_loop	x30
 	.endif // \bhb == BHB_MITIGATION_LOOP
 
+	.if	\bhb == BHB_MITIGATION_INSN
+	clearbhb
+	isb
+	.endif // \bhb == BHB_MITIGATION_INSN
+
 	.if	\kpti == 1
 	/*
 	 * Defend against branch aliasing attacks by pushing a dummy
@@ -919,6 +925,7 @@ SYM_CODE_START_NOALIGN(tramp_vectors)
 #ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
 	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_LOOP
 	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_FW
+	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_INSN
 #endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
 	generate_tramp_vector	kpti=1, bhb=BHB_MITIGATION_NONE
 SYM_CODE_END(tramp_vectors)
@@ -981,6 +988,7 @@ SYM_CODE_START(__bp_harden_el1_vectors)
 #ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
 	generate_el1_vector	bhb=BHB_MITIGATION_LOOP
 	generate_el1_vector	bhb=BHB_MITIGATION_FW
+	generate_el1_vector	bhb=BHB_MITIGATION_INSN
 #endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
 SYM_CODE_END(__bp_harden_el1_vectors)
 	.popsection

arch/arm64/kernel/proton-pack.c

@@ -824,6 +824,7 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
  * - Mitigated by a branchy loop a CPU specific number of times, and listed
  *   in our "loop mitigated list".
  * - Mitigated in software by the firmware Spectre v2 call.
+ * - Has the ClearBHB instruction to perform the mitigation.
  * - Has the 'Exception Clears Branch History Buffer' (ECBHB) feature, so no
  *   software mitigation in the vectors is needed.
  * - Has CSV2.3, so is unaffected.
@@ -949,6 +950,9 @@ bool is_spectre_bhb_affected(const struct arm64_cpu_capabilities *entry,
 	if (supports_csv2p3(scope))
 		return false;
 
+	if (supports_clearbhb(scope))
+		return true;
+
 	if (spectre_bhb_loop_affected(scope))
 		return true;
 
@@ -987,6 +991,8 @@ static int kvm_bhb_get_vecs_size(const char *start)
 		 start == __spectre_bhb_loop_k24 ||
 		 start == __spectre_bhb_loop_k32)
 		return __SPECTRE_BHB_LOOP_SZ;
+	else if (start == __spectre_bhb_clearbhb)
+		return __SPECTRE_BHB_CLEARBHB_SZ;
 
 	return 0;
 }
@@ -1027,6 +1033,7 @@ static void kvm_setup_bhb_slot(const char *hyp_vecs_start)
 #define __spectre_bhb_loop_k8 NULL
 #define __spectre_bhb_loop_k24 NULL
 #define __spectre_bhb_loop_k32 NULL
+#define __spectre_bhb_clearbhb NULL
 
 static void kvm_setup_bhb_slot(const char *hyp_vecs_start) { }
 #endif /* CONFIG_KVM */
@@ -1045,6 +1052,11 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 	} else if (cpu_mitigations_off()) {
 		pr_info_once("spectre-bhb mitigation disabled by command line option\n");
 	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
+		state = SPECTRE_MITIGATED;
+	} else if (supports_clearbhb(SCOPE_LOCAL_CPU)) {
+		kvm_setup_bhb_slot(__spectre_bhb_clearbhb);
+		this_cpu_set_vectors(EL1_VECTOR_BHB_CLEAR_INSN);
+
 		state = SPECTRE_MITIGATED;
 	} else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) {
 		switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) {

arch/arm64/kvm/hyp/smccc_wa.S

@@ -96,3 +96,12 @@ SYM_DATA_START(__spectre_bhb_loop_k32)
 1:	.org __spectre_bhb_loop_k32 + __SPECTRE_BHB_LOOP_SZ
 	.org 1b
 SYM_DATA_END(__spectre_bhb_loop_k32)
+
+	.global	__spectre_bhb_clearbhb
+SYM_DATA_START(__spectre_bhb_clearbhb)
+	esb
+	clearbhb
+	isb
+1:	.org __spectre_bhb_clearbhb + __SPECTRE_BHB_CLEARBHB_SZ
+	.org 1b
+SYM_DATA_END(__spectre_bhb_clearbhb)