arm64: Add skeleton to harden the branch predictor against aliasing attacks

Aliasing attacks against CPU branch predictors can allow the attacks to
redirect speculative control flow on some CPUs and potentially divulge
information from one context to another.

This patch adds initial skeleton code behind a new Kconfig option to
enable implementation-specific mitigations against these attacks for
CPUs that are affected.

Change-Id: I07fba1943dd63df8951bf68fac947666100e5559
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 0f15adbb2861ce6f75ccfc5a92b19eae0ef327d0
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[sramana@codeaurora.org: Fix merge conflicts and make it
compilable on msm-4.4]
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>

arch/arm64/Kconfig

@@ -809,6 +809,23 @@ config UNMAP_KERNEL_AT_EL0
 
 	  If unsure, say Y.
 
+config HARDEN_BRANCH_PREDICTOR
+	bool "Harden the branch predictor against aliasing attacks" if EXPERT
+	default y
+	help
+	  Speculation attacks against some high-performance processors rely on
+	  being able to manipulate the branch predictor for a victim context by
+	  executing aliasing branches in the attacker context.  Such attacks
+	  can be partially mitigated against by clearing internal branch
+	  predictor state and limiting the prediction logic in some situations.
+
+	  This config option will take CPU-specific actions to harden the
+	  branch predictor against aliasing attacks and may rely on specific
+	  instruction sequences or control bits being set by the system
+	  firmware.
+
+	  If unsure, say Y.
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on COMPAT

arch/arm64/include/asm/cpufeature.h

@@ -36,8 +36,9 @@
 
 #define ARM64_WORKAROUND_CAVIUM_27456		11
 #define ARM64_HAS_VIRT_HOST_EXTN		12
-#define ARM64_UNMAP_KERNEL_AT_EL0		23
+#define ARM64_HARDEN_BRANCH_PREDICTOR		13
-#define ARM64_NCAPS				24
+#define ARM64_UNMAP_KERNEL_AT_EL0		14
+#define ARM64_NCAPS				15
 
 #ifndef __ASSEMBLY__
 

arch/arm64/include/asm/mmu.h

@@ -20,6 +20,10 @@
 #define TTBR_ASID_MASK	(UL(0xffff) << 48)
 
 #ifndef __ASSEMBLY__
+#include <linux/smp.h>
+
+#include <asm/cpufeature.h>
+#include <asm/percpu.h>
 
 typedef struct {
 	atomic64_t	id;
@@ -39,6 +43,43 @@ static inline bool arm64_kernel_unmapped_at_el0(void)
 	       cpus_have_cap(ARM64_UNMAP_KERNEL_AT_EL0);
 }
 
+typedef void (*bp_hardening_cb_t)(void);
+
+struct bp_hardening_data {
+	int			hyp_vectors_slot;
+	bp_hardening_cb_t	fn;
+};
+
+#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
+extern char __bp_harden_hyp_vecs_start[], __bp_harden_hyp_vecs_end[];
+
+DECLARE_PER_CPU_READ_MOSTLY(struct bp_hardening_data, bp_hardening_data);
+
+static inline struct bp_hardening_data *arm64_get_bp_hardening_data(void)
+{
+	return this_cpu_ptr(&bp_hardening_data);
+}
+
+static inline void arm64_apply_bp_hardening(void)
+{
+	struct bp_hardening_data *d;
+
+	if (!cpus_have_cap(ARM64_HARDEN_BRANCH_PREDICTOR))
+		return;
+
+	d = arm64_get_bp_hardening_data();
+	if (d->fn)
+		d->fn();
+}
+#else
+static inline struct bp_hardening_data *arm64_get_bp_hardening_data(void)
+{
+	return NULL;
+}
+
+static inline void arm64_apply_bp_hardening(void)	{ }
+#endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
+
 extern void paging_init(void);
 extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);
 extern void init_mem_pgprot(void);

arch/arm64/include/asm/sysreg.h

@@ -112,6 +112,7 @@
 #define ID_AA64ISAR0_AES_SHIFT		4
 
 /* id_aa64pfr0 */
+#define ID_AA64PFR0_CSV2_SHIFT		56
 #define ID_AA64PFR0_GIC_SHIFT		24
 #define ID_AA64PFR0_ASIMD_SHIFT		20
 #define ID_AA64PFR0_FP_SHIFT		16

arch/arm64/kernel/Makefile

@@ -49,6 +49,10 @@ arm64-obj-$(CONFIG_HIBERNATION)		+= hibernate.o hibernate-asm.o
 arm64-obj-$(CONFIG_ARM64_ACPI_PARKING_PROTOCOL)	+= acpi_parking_protocol.o
 arm64-obj-$(CONFIG_PARAVIRT)		+= paravirt.o
 
+ifeq ($(CONFIG_KVM),y)
+arm64-obj-$(CONFIG_HARDEN_BRANCH_PREDICTOR)	+= bpi.o
+endif
+
 obj-y					+= $(arm64-obj-y) vdso/ probes/
 obj-m					+= $(arm64-obj-m)
 head-y					:= head.o

arch/arm64/kernel/bpi.S

@@ -0,0 +1,55 @@
+/*
+ * Contains CPU specific branch predictor invalidation sequences
+ *
+ * Copyright (C) 2018 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+
+.macro ventry target
+	.rept 31
+	nop
+	.endr
+	b	\target
+.endm
+
+.macro vectors target
+	ventry \target + 0x000
+	ventry \target + 0x080
+	ventry \target + 0x100
+	ventry \target + 0x180
+
+	ventry \target + 0x200
+	ventry \target + 0x280
+	ventry \target + 0x300
+	ventry \target + 0x380
+
+	ventry \target + 0x400
+	ventry \target + 0x480
+	ventry \target + 0x500
+	ventry \target + 0x580
+
+	ventry \target + 0x600
+	ventry \target + 0x680
+	ventry \target + 0x700
+	ventry \target + 0x780
+.endm
+
+	.align	11
+ENTRY(__bp_harden_hyp_vecs_start)
+	.rept 4
+	vectors __kvm_hyp_vector
+	.endr
+ENTRY(__bp_harden_hyp_vecs_end)

arch/arm64/kernel/cpu_errata.c

@@ -29,6 +29,82 @@ is_affected_midr_range(const struct arm64_cpu_capabilities *entry)
 				       entry->midr_range_max);
 }
 
+#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
+#include <asm/mmu_context.h>
+#include <asm/cacheflush.h>
+
+DEFINE_PER_CPU_READ_MOSTLY(struct bp_hardening_data, bp_hardening_data);
+
+#ifdef CONFIG_KVM
+static void __copy_hyp_vect_bpi(int slot, const char *hyp_vecs_start,
+				const char *hyp_vecs_end)
+{
+	void *dst = lm_alias(__bp_harden_hyp_vecs_start + slot * SZ_2K);
+	int i;
+
+	for (i = 0; i < SZ_2K; i += 0x80)
+		memcpy(dst + i, hyp_vecs_start, hyp_vecs_end - hyp_vecs_start);
+
+	flush_icache_range((uintptr_t)dst, (uintptr_t)dst + SZ_2K);
+}
+
+static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
+				      const char *hyp_vecs_start,
+				      const char *hyp_vecs_end)
+{
+	static int last_slot = -1;
+	static DEFINE_SPINLOCK(bp_lock);
+	int cpu, slot = -1;
+
+	spin_lock(&bp_lock);
+	for_each_possible_cpu(cpu) {
+		if (per_cpu(bp_hardening_data.fn, cpu) == fn) {
+			slot = per_cpu(bp_hardening_data.hyp_vectors_slot, cpu);
+			break;
+		}
+	}
+
+	if (slot == -1) {
+		last_slot++;
+		BUG_ON(((__bp_harden_hyp_vecs_end - __bp_harden_hyp_vecs_start)
+			/ SZ_2K) <= last_slot);
+		slot = last_slot;
+		__copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end);
+	}
+
+	__this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot);
+	__this_cpu_write(bp_hardening_data.fn, fn);
+	spin_unlock(&bp_lock);
+}
+#else
+
+static void __maybe_unused __install_bp_hardening_cb(bp_hardening_cb_t fn,
+				      const char *hyp_vecs_start,
+				      const char *hyp_vecs_end)
+{
+	__this_cpu_write(bp_hardening_data.fn, fn);
+}
+#endif	/* CONFIG_KVM */
+
+static void __maybe_unused install_bp_hardening_cb(
+				const struct arm64_cpu_capabilities *entry,
+				bp_hardening_cb_t fn,
+				const char *hyp_vecs_start,
+				const char *hyp_vecs_end)
+{
+	u64 pfr0;
+
+	if (!entry->matches(entry))
+		return;
+
+	pfr0 = read_cpuid(SYS_ID_AA64PFR0_EL1);
+	if (cpuid_feature_extract_unsigned_field(pfr0, ID_AA64PFR0_CSV2_SHIFT))
+		return;
+
+	__install_bp_hardening_cb(fn, hyp_vecs_start, hyp_vecs_end);
+}
+#endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
+
 #define MIDR_RANGE(model, min, max) \
 	.matches = is_affected_midr_range, \
 	.midr_model = model, \

arch/arm64/kernel/cpufeature.c

@@ -92,6 +92,7 @@ static struct arm64_ftr_bits ftr_id_aa64isar0[] = {
 static struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
 	ARM64_FTR_BITS(FTR_STRICT, FTR_EXACT, 32, 32, 0),
 	ARM64_FTR_BITS(FTR_STRICT, FTR_EXACT, 28, 4, 0),
+	ARM64_FTR_BITS(FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV2_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_STRICT, FTR_EXACT, ID_AA64PFR0_GIC_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
 	ARM64_FTR_BITS(FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_FP_SHIFT, 4, ID_AA64PFR0_FP_NI),

arch/arm64/kernel/entry.S

@@ -686,13 +686,15 @@ el0_ia:
 	 * Instruction abort handling
 	 */
 	mrs	x26, far_el1
-	// enable interrupts before calling the main handler
+	enable_dbg
-	enable_dbg_and_irq
+#ifdef CONFIG_TRACE_IRQFLAGS
+	bl	trace_hardirqs_off
+#endif
 	ct_user_exit
 	mov	x0, x26
 	mov	x1, x25
 	mov	x2, sp
-	bl	do_mem_abort
+	bl	do_el0_ia_bp_hardening
 	b	ret_to_user
 el0_fpsimd_acc:
 	/*

arch/arm64/mm/context.c

@@ -206,6 +206,8 @@ asmlinkage void post_ttbr_update_workaround(void)
 			"ic iallu; dsb nsh; isb",
 			ARM64_WORKAROUND_CAVIUM_27456,
 			CONFIG_CAVIUM_ERRATUM_27456));
+
+	arm64_apply_bp_hardening();
 }
 
 static int asids_init(void)

arch/arm64/mm/fault.c

@@ -609,6 +609,22 @@ asmlinkage void __exception do_mem_abort(unsigned long addr, unsigned int esr,
 	arm64_notify_die("", regs, &info, esr);
 }
 
+asmlinkage void __exception do_el0_ia_bp_hardening(unsigned long addr,
+						   unsigned int esr,
+						   struct pt_regs *regs)
+{
+	/*
+	 * We've taken an instruction abort from userspace and not yet
+	 * re-enabled IRQs. If the address is a kernel address, apply
+	 * BP hardening prior to enabling IRQs and pre-emption.
+	 */
+	if (addr > TASK_SIZE)
+		arm64_apply_bp_hardening();
+
+	local_irq_enable();
+	do_mem_abort(addr, esr, regs);
+}
+
 /*
  * Handle stack alignment exceptions.
  */