[PATCH v2 07/11] arm64: Add skeleton to harden the branch predictor against aliasing attacks

Will Deacon
2018-01-05T13:12:37Z

Aliasing attacks against CPU branch predictors can allow an attacker 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.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
arch/arm64/Kconfig | 17 +++++++++
arch/arm64/include/asm/cpucaps.h | 3 +-
arch/arm64/include/asm/mmu.h | 37 ++++++++++++++++++++
arch/arm64/include/asm/sysreg.h | 1 +
arch/arm64/kernel/Makefile | 4 +++
arch/arm64/kernel/bpi.S | 55 +++++++++++++++++++++++++++++
arch/arm64/kernel/cpu_errata.c | 74 ++++++++++++++++++++++++++++++++++++++++
arch/arm64/kernel/cpufeature.c | 1 +
arch/arm64/mm/context.c | 2 ++
arch/arm64/mm/fault.c | 1 +
10 files changed, 194 insertions(+), 1 deletion(-)
create mode 100644 arch/arm64/kernel/bpi.S

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index efaaa3a66b95..cea44b95187c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -845,6 +845,23 @@ 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
diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index b4537ffd1018..51616e77fe6b 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -42,7 +42,8 @@
#define ARM64_HAS_DCPOP 21
#define ARM64_SVE 22
#define ARM64_UNMAP_KERNEL_AT_EL0 23
+#define ARM64_HARDEN_BRANCH_PREDICTOR 24

-#define ARM64_NCAPS 24
+#define ARM64_NCAPS 25

#endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 6f7bdb89817f..6dd83d75b82a 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -41,6 +41,43 @@ static inline bool arm64_kernel_unmapped_at_el0(void)
cpus_have_const_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_const_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 bootmem_init(void);
extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index ae519bbd3f9e..871744973ece 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -438,6 +438,7 @@

/* id_aa64pfr0 */
#define ID_AA64PFR0_CSV3_SHIFT 60
+#define ID_AA64PFR0_CSV2_SHIFT 56
#define ID_AA64PFR0_SVE_SHIFT 32
#define ID_AA64PFR0_GIC_SHIFT 24
#define ID_AA64PFR0_ASIMD_SHIFT 20
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 067baace74a0..0c760db04858 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -53,6 +53,10 @@ arm64-obj-$(CONFIG_ARM64_RELOC_TEST) += arm64-reloc-test.o
arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.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
diff --git a/arch/arm64/kernel/bpi.S b/arch/arm64/kernel/bpi.S
new file mode 100644
index 000000000000..06a931eb2673
--- /dev/null
+++ b/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)
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 0e27f86ee709..16ea5c6f314e 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -46,6 +46,80 @@ static int cpu_enable_trap_ctr_access(void *__unused)
return 0;
}

+#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 __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 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, SCOPE_LOCAL_CPU))
+ return;
+
+ pfr0 = read_cpuid(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) \
.def_scope = SCOPE_LOCAL_CPU, \
.matches = is_affected_midr_range, \
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 55712ab4e3bf..9d4d82c11528 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -146,6 +146,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {

static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
+ ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV2_SHIFT, 4, 0),
ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
index 5f7097d0cd12..d99b36555a16 100644
--- a/arch/arm64/mm/context.c
+++ b/arch/arm64/mm/context.c
@@ -246,6 +246,8 @@ 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)
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 22168cd0dde7..5203b6040cb6 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -318,6 +318,7 @@ static void __do_user_fault(struct task_struct *tsk, unsigned long addr,
lsb = PAGE_SHIFT;
si.si_addr_lsb = lsb;

+ arm64_apply_bp_hardening();
force_sig_info(sig, &si, tsk);
}

--
2.1.4