arm64: Implement branch predictor hardening for affected Cortex-A CPUs

Cortex-A57, A72, A73 and A75 are susceptible to branch predictor aliasing
and can theoretically be attacked by malicious code.

This patch implements a PSCI-based mitigation for these CPUs when available.
The call into firmware will invalidate the branch predictor state, preventing
any malicious entries from affecting other victim contexts.
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/bpi.S

@@ -53,3 +53,27 @@ ENTRY(__bp_harden_hyp_vecs_start)
 	vectors __kvm_hyp_vector
 	.endr
 ENTRY(__bp_harden_hyp_vecs_end)
+ENTRY(__psci_hyp_bp_inval_start)
+	sub	sp, sp, #(8 * 18)
+	stp	x16, x17, [sp, #(16 * 0)]
+	stp	x14, x15, [sp, #(16 * 1)]
+	stp	x12, x13, [sp, #(16 * 2)]
+	stp	x10, x11, [sp, #(16 * 3)]
+	stp	x8, x9, [sp, #(16 * 4)]
+	stp	x6, x7, [sp, #(16 * 5)]
+	stp	x4, x5, [sp, #(16 * 6)]
+	stp	x2, x3, [sp, #(16 * 7)]
+	stp	x0, x1, [sp, #(16 * 8)]
+	mov	x0, #0x84000000
+	smc	#0
+	ldp	x16, x17, [sp, #(16 * 0)]
+	ldp	x14, x15, [sp, #(16 * 1)]
+	ldp	x12, x13, [sp, #(16 * 2)]
+	ldp	x10, x11, [sp, #(16 * 3)]
+	ldp	x8, x9, [sp, #(16 * 4)]
+	ldp	x6, x7, [sp, #(16 * 5)]
+	ldp	x4, x5, [sp, #(16 * 6)]
+	ldp	x2, x3, [sp, #(16 * 7)]
+	ldp	x0, x1, [sp, #(16 * 8)]
+	add	sp, sp, #(8 * 18)
+ENTRY(__psci_hyp_bp_inval_end)

arch/arm64/kernel/cpu_errata.c

@@ -53,6 +53,8 @@ static int cpu_enable_trap_ctr_access(void *__unused)
 DEFINE_PER_CPU_READ_MOSTLY(struct bp_hardening_data, bp_hardening_data);
 
 #ifdef CONFIG_KVM
+extern char __psci_hyp_bp_inval_start[], __psci_hyp_bp_inval_end[];
+
 static void __copy_hyp_vect_bpi(int slot, const char *hyp_vecs_start,
 				const char *hyp_vecs_end)
 {
@@ -94,6 +96,9 @@ static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
 	spin_unlock(&bp_lock);
 }
 #else
+#define __psci_hyp_bp_inval_start	NULL
+#define __psci_hyp_bp_inval_end		NULL
+
 static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
 				      const char *hyp_vecs_start,
 				      const char *hyp_vecs_end)
@@ -118,6 +123,21 @@ static void  install_bp_hardening_cb(const struct arm64_cpu_capabilities *entry,
 
 	__install_bp_hardening_cb(fn, hyp_vecs_start, hyp_vecs_end);
 }
+
+#include <linux/psci.h>
+
+static int enable_psci_bp_hardening(void *data)
+{
+	const struct arm64_cpu_capabilities *entry = data;
+
+	if (psci_ops.get_version)
+		install_bp_hardening_cb(entry,
+				       (bp_hardening_cb_t)psci_ops.get_version,
+				       __psci_hyp_bp_inval_start,
+				       __psci_hyp_bp_inval_end);
+
+	return 0;
+}
 #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
 
 #define MIDR_RANGE(model, min, max) \
@@ -260,6 +280,28 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 		.capability = ARM64_WORKAROUND_858921,
 		MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
 	},
+#endif
+#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
+	{
+		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A57),
+		.enable = enable_psci_bp_hardening,
+	},
+	{
+		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A72),
+		.enable = enable_psci_bp_hardening,
+	},
+	{
+		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
+		.enable = enable_psci_bp_hardening,
+	},
+	{
+		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A75),
+		.enable = enable_psci_bp_hardening,
+	},
 #endif
 	{
 	}