Implement the SSBD (CVE-2018-3639) workaround on arm64

This calls into the Arm Trusted Firmware to enable and disable the
workaround for the Speculative Store Bypass Disable (SSBD) issue, also
known as Spectre Variant 4.

As this may have a large performance overhead, and how exploitable SSBD is
is unknown we follow the Linux lead of allowing the administrator to select
between always on, always off, or only enabled in the kernel, with the
latter being the default.

PR:		228955
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D15819

sys/arm64/arm64/cpu_errata.c

@@ -38,6 +38,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/param.h>
 #include <sys/kernel.h>
 #include <sys/pcpu.h>
+#include <sys/systm.h>
 
 #include <machine/cpu.h>
 
@@ -50,7 +51,14 @@ struct cpu_quirks {
 	u_int		midr_value;
 };
 
+static enum {
+	SSBD_FORCE_ON,
+	SSBD_FORCE_OFF,
+	SSBD_KERNEL,
+} ssbd_method = SSBD_KERNEL;
+
 static cpu_quirk_install install_psci_bp_hardening;
+static cpu_quirk_install install_ssbd_workaround;
 
 static struct cpu_quirks cpu_quirks[] = {
 	{
@@ -79,6 +87,11 @@ static struct cpu_quirks cpu_quirks[] = {
 		    CPU_ID_RAW(CPU_IMPL_CAVIUM, CPU_PART_THUNDERX2, 0,0),
 		.quirk_install = install_psci_bp_hardening,
 	},
+	{
+		.midr_mask = 0,
+		.midr_value = 0,
+		.quirk_install = install_ssbd_workaround,
+	},
 };
 
 static void
@@ -91,6 +104,40 @@ install_psci_bp_hardening(void)
 	PCPU_SET(bp_harden, smccc_arch_workaround_1);
 }
 
+static void
+install_ssbd_workaround(void)
+{
+	char *env;
+
+	if (PCPU_GET(cpuid) == 0) {
+		env = kern_getenv("kern.cfg.ssbd");
+		if (env != NULL) {
+			if (strcmp(env, "force-on") == 0) {
+				ssbd_method = SSBD_FORCE_ON;
+			} else if (strcmp(env, "force-off") == 0) {
+				ssbd_method = SSBD_FORCE_OFF;
+			}
+		}
+	}
+
+	/* Enable the workaround on this CPU if it's enabled in the firmware */
+	if (smccc_arch_features(SMCCC_ARCH_WORKAROUND_2) != SMCCC_RET_SUCCESS)
+		return;
+
+	switch(ssbd_method) {
+	case SSBD_FORCE_ON:
+		smccc_arch_workaround_2(true);
+		break;
+	case SSBD_FORCE_OFF:
+		smccc_arch_workaround_2(false);
+		break;
+	case SSBD_KERNEL:
+	default:
+		PCPU_SET(ssbd, smccc_arch_workaround_2);
+		break;
+	}
+}
+
 void
 install_cpu_errata(void)
 {

sys/arm64/arm64/exception.S

@@ -66,6 +66,14 @@ __FBSDID("$FreeBSD$");
 	stp	x18,  lr, [sp, #(TF_SP)]
 	mrs	x18, tpidr_el1
 	add	x29, sp, #(TF_SIZE)
+.if \el == 0
+	/* Apply the SSBD (CVE-2018-3639) workaround if needed */
+	ldr	x1, [x18, #PC_SSBD]
+	cbz	x1, 1f
+	mov	w0, #1
+	blr	x1
+1:
+.endif
 .endm
 
 .macro	restore_registers el
@@ -75,6 +83,14 @@ __FBSDID("$FreeBSD$");
 	 * Disable interrupts, x18 may change in the interrupt exception
 	 * handler.  For EL0 exceptions, do_ast already did this.
 	 */
+.endif
+.if \el == 0
+	/* Remove the SSBD (CVE-2018-3639) workaround if needed */
+	ldr	x1, [x18, #PC_SSBD]
+	cbz	x1, 1f
+	mov	w0, #0
+	blr	x1
+1:
 .endif
 	ldp	x18,  lr, [sp, #(TF_SP)]
 	ldp	x10, x11, [sp, #(TF_ELR)]

sys/arm64/arm64/genassym.c

@@ -43,6 +43,7 @@ ASSYM(TDF_NEEDRESCHED, TDF_NEEDRESCHED);
 ASSYM(PCPU_SIZE, sizeof(struct pcpu));
 ASSYM(PC_CURPCB, offsetof(struct pcpu, pc_curpcb));
 ASSYM(PC_CURTHREAD, offsetof(struct pcpu, pc_curthread));
+ASSYM(PC_SSBD, offsetof(struct pcpu, pc_ssbd));
 
 /* Size of pcb, rounded to keep stack alignment */
 ASSYM(PCB_SIZE, roundup2(sizeof(struct pcb), STACKALIGNBYTES + 1));

sys/arm64/include/pcpu.h

@@ -36,13 +36,15 @@
 #define	ALT_STACK_SIZE	128
 
 typedef int (*pcpu_bp_harden)(void);
+typedef int (*pcpu_ssbd)(bool);
 
 #define	PCPU_MD_FIELDS							\
 	u_int	pc_acpi_id;	/* ACPI CPU id */		\
 	u_int	pc_midr;	/* stored MIDR value */	\
 	uint64_t pc_clock;						\
 	pcpu_bp_harden pc_bp_harden;					\
-	char __pad[233]
+	pcpu_ssbd pc_ssbd;						\
+	char __pad[225]
 
 #ifdef _KERNEL
 

sys/dev/psci/smccc.c

@@ -91,3 +91,12 @@ smccc_arch_workaround_1(void)
 	    ("SMCCC arch workaround 1 called with an invalid SMCCC interface"));
 	return (psci_call(SMCCC_ARCH_WORKAROUND_1, 0, 0, 0));
 }
+
+int
+smccc_arch_workaround_2(bool enable)
+{
+
+	KASSERT(smccc_version != SMCCC_VERSION_1_0,
+	    ("SMCCC arch workaround 2 called with an invalid SMCCC interface"));
+	return (psci_call(SMCCC_ARCH_WORKAROUND_2, enable ? 1 : 0, 0, 0));
+}

sys/dev/psci/smccc.h

@@ -59,6 +59,8 @@
     SMCCC_FUNC_ID(SMCCC_FAST_CALL, SMCCC_32BIT_CALL, 0, 1)
 #define	SMCCC_ARCH_WORKAROUND_1						\
     SMCCC_FUNC_ID(SMCCC_FAST_CALL, SMCCC_32BIT_CALL, 0, 0x8000)
+#define	SMCCC_ARCH_WORKAROUND_2						\
+    SMCCC_FUNC_ID(SMCCC_FAST_CALL, SMCCC_32BIT_CALL, 0, 0x7fff)
 
 /* The return values from ARM DEN 0070A. */
 #define	SMCCC_RET_SUCCESS		0
@@ -67,6 +69,7 @@
 
 int32_t smccc_arch_features(uint32_t);
 int smccc_arch_workaround_1(void);
+int smccc_arch_workaround_2(bool);
 
 
 #endif /* _PSCI_SMCCC_H_ */