[PATCH AUTOSEL 4.14 14/21] ARM: spectre-v2: per-CPU vtables to work around big.Little systems

[Sasha Levin]

From: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>

[ Upstream commit 383fb3ee8024d596f488d2dbaf45e572897acbdb ]

In big.Little systems, some CPUs require the Spectre workarounds in
paths such as the context switch, but other CPUs do not.  In order
to handle these differences, we need per-CPU vtables.

We are unable to use the kernel's per-CPU variables to support this
as per-CPU is not initialised at times when we need access to the
vtables, so we have to use an array indexed by logical CPU number.

We use an array-of-pointers to avoid having function pointers in
the kernel's read/write .data section.

Reviewed-by: Julien Thierry <julien.thierry@xxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/arm/include/asm/proc-fns.h | 23 +++++++++++++++++++++++
 arch/arm/kernel/setup.c         |  5 +++++
 arch/arm/kernel/smp.c           | 31 +++++++++++++++++++++++++++++++
 arch/arm/mm/proc-v7-bugs.c      | 17 ++---------------
 4 files changed, 61 insertions(+), 15 deletions(-)

diff --git a/arch/arm/include/asm/proc-fns.h b/arch/arm/include/asm/proc-fns.h
index c259cc49c641..e1b6f280ab08 100644
--- a/arch/arm/include/asm/proc-fns.h
+++ b/arch/arm/include/asm/proc-fns.h
@@ -104,12 +104,35 @@ extern void cpu_do_resume(void *);
 #else
 
 extern struct processor processor;
+#if defined(CONFIG_BIG_LITTLE) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
+#include <linux/smp.h>
+/*
+ * This can't be a per-cpu variable because we need to access it before
+ * per-cpu has been initialised.  We have a couple of functions that are
+ * called in a pre-emptible context, and so can't use smp_processor_id()
+ * there, hence PROC_TABLE().  We insist in init_proc_vtable() that the
+ * function pointers for these are identical across all CPUs.
+ */
+extern struct processor *cpu_vtable[];
+#define PROC_VTABLE(f)			cpu_vtable[smp_processor_id()]->f
+#define PROC_TABLE(f)			cpu_vtable[0]->f
+static inline void init_proc_vtable(const struct processor *p)
+{
+	unsigned int cpu = smp_processor_id();
+	*cpu_vtable[cpu] = *p;
+	WARN_ON_ONCE(cpu_vtable[cpu]->dcache_clean_area !=
+		     cpu_vtable[0]->dcache_clean_area);
+	WARN_ON_ONCE(cpu_vtable[cpu]->set_pte_ext !=
+		     cpu_vtable[0]->set_pte_ext);
+}
+#else
 #define PROC_VTABLE(f)			processor.f
 #define PROC_TABLE(f)			processor.f
 static inline void init_proc_vtable(const struct processor *p)
 {
 	processor = *p;
 }
+#endif
 
 #define cpu_proc_init			PROC_VTABLE(_proc_init)
 #define cpu_check_bugs			PROC_VTABLE(check_bugs)
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index c1588e31aa83..a6d27284105a 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -115,6 +115,11 @@ EXPORT_SYMBOL(elf_hwcap2);
 
 #ifdef MULTI_CPU
 struct processor processor __ro_after_init;
+#if defined(CONFIG_BIG_LITTLE) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
+struct processor *cpu_vtable[NR_CPUS] = {
+	[0] = &processor,
+};
+#endif
 #endif
 #ifdef MULTI_TLB
 struct cpu_tlb_fns cpu_tlb __ro_after_init;
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index e61af0600133..f6b1c9d2e178 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -42,6 +42,7 @@
 #include <asm/mmu_context.h>
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
+#include <asm/procinfo.h>
 #include <asm/processor.h>
 #include <asm/sections.h>
 #include <asm/tlbflush.h>
@@ -102,6 +103,30 @@ static unsigned long get_arch_pgd(pgd_t *pgd)
 #endif
 }
 
+#if defined(CONFIG_BIG_LITTLE) && defined(CONFIG_HARDEN_BRANCH_PREDICTOR)
+static int secondary_biglittle_prepare(unsigned int cpu)
+{
+	if (!cpu_vtable[cpu])
+		cpu_vtable[cpu] = kzalloc(sizeof(*cpu_vtable[cpu]), GFP_KERNEL);
+
+	return cpu_vtable[cpu] ? 0 : -ENOMEM;
+}
+
+static void secondary_biglittle_init(void)
+{
+	init_proc_vtable(lookup_processor(read_cpuid_id())->proc);
+}
+#else
+static int secondary_biglittle_prepare(unsigned int cpu)
+{
+	return 0;
+}
+
+static void secondary_biglittle_init(void)
+{
+}
+#endif
+
 int __cpu_up(unsigned int cpu, struct task_struct *idle)
 {
 	int ret;
@@ -109,6 +134,10 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 	if (!smp_ops.smp_boot_secondary)
 		return -ENOSYS;
 
+	ret = secondary_biglittle_prepare(cpu);
+	if (ret)
+		return ret;
+
 	/*
 	 * We need to tell the secondary core where to find
 	 * its stack and the page tables.
@@ -360,6 +389,8 @@ asmlinkage void secondary_start_kernel(void)
 	struct mm_struct *mm = &init_mm;
 	unsigned int cpu;
 
+	secondary_biglittle_init();
+
 	/*
 	 * The identity mapping is uncached (strongly ordered), so
 	 * switch away from it before attempting any exclusive accesses.
diff --git a/arch/arm/mm/proc-v7-bugs.c b/arch/arm/mm/proc-v7-bugs.c
index 5544b82a2e7a..9a07916af8dd 100644
--- a/arch/arm/mm/proc-v7-bugs.c
+++ b/arch/arm/mm/proc-v7-bugs.c
@@ -52,8 +52,6 @@ static void cpu_v7_spectre_init(void)
 	case ARM_CPU_PART_CORTEX_A17:
 	case ARM_CPU_PART_CORTEX_A73:
 	case ARM_CPU_PART_CORTEX_A75:
-		if (processor.switch_mm != cpu_v7_bpiall_switch_mm)
-			goto bl_error;
 		per_cpu(harden_branch_predictor_fn, cpu) =
 			harden_branch_predictor_bpiall;
 		spectre_v2_method = "BPIALL";
@@ -61,8 +59,6 @@ static void cpu_v7_spectre_init(void)
 
 	case ARM_CPU_PART_CORTEX_A15:
 	case ARM_CPU_PART_BRAHMA_B15:
-		if (processor.switch_mm != cpu_v7_iciallu_switch_mm)
-			goto bl_error;
 		per_cpu(harden_branch_predictor_fn, cpu) =
 			harden_branch_predictor_iciallu;
 		spectre_v2_method = "ICIALLU";
@@ -88,11 +84,9 @@ static void cpu_v7_spectre_init(void)
 					  ARM_SMCCC_ARCH_WORKAROUND_1, &res);
 			if ((int)res.a0 != 0)
 				break;
-			if (processor.switch_mm != cpu_v7_hvc_switch_mm && cpu)
-				goto bl_error;
 			per_cpu(harden_branch_predictor_fn, cpu) =
 				call_hvc_arch_workaround_1;
-			processor.switch_mm = cpu_v7_hvc_switch_mm;
+			cpu_do_switch_mm = cpu_v7_hvc_switch_mm;
 			spectre_v2_method = "hypervisor";
 			break;
 
@@ -101,11 +95,9 @@ static void cpu_v7_spectre_init(void)
 					  ARM_SMCCC_ARCH_WORKAROUND_1, &res);
 			if ((int)res.a0 != 0)
 				break;
-			if (processor.switch_mm != cpu_v7_smc_switch_mm && cpu)
-				goto bl_error;
 			per_cpu(harden_branch_predictor_fn, cpu) =
 				call_smc_arch_workaround_1;
-			processor.switch_mm = cpu_v7_smc_switch_mm;
+			cpu_do_switch_mm = cpu_v7_smc_switch_mm;
 			spectre_v2_method = "firmware";
 			break;
 
@@ -119,11 +111,6 @@ static void cpu_v7_spectre_init(void)
 	if (spectre_v2_method)
 		pr_info("CPU%u: Spectre v2: using %s workaround\n",
 			smp_processor_id(), spectre_v2_method);
-	return;
-
-bl_error:
-	pr_err("CPU%u: Spectre v2: incorrect context switching function, system vulnerable\n",
-		cpu);
 }
 #else
 static void cpu_v7_spectre_init(void)
-- 
2.17.1