Subject: [RFC PATCH 07/13] powerpc/dexcr: Add sysctl entry for SBHE system override

From: Benjamin Gray <[email-masked]>
Date: Mon, 28 Nov 2022 13:44:52 +1100

The DEXCR Speculative Branch Hint Enable (SBHE) aspect controls whether
the hints provided by BO field of Branch instructions are obeyed during
speculative execution.

SBHE behaviour per ISA 3.1B:

0:	The hints provided by BO field of Branch instructions may be
	ignored during speculative execution

1:	The hints provided by BO field of Branch instructions are obeyed
	during speculative execution

Add a sysctl entry to allow changing this aspect globally in the system
at runtime:

	/proc/sys/kernel/speculative_branch_hint_enable

Three values are supported:

-1:	Disable DEXCR SBHE sysctl override
 0:	Override and set DEXCR[SBHE] aspect to 0
 1:	Override and set DEXCR[SBHE] aspect to 1

Internally, introduces a mechanism to apply arbitrary system wide
overrides on top of the prctl() config.

Signed-off-by: Benjamin Gray <[email-masked]>
---
 arch/powerpc/kernel/dexcr.c | 125 ++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)

diff --git a/arch/powerpc/kernel/dexcr.c b/arch/powerpc/kernel/dexcr.c
index 9290beed722a..8239bcc92026 100644
--- a/arch/powerpc/kernel/dexcr.c
+++ b/arch/powerpc/kernel/dexcr.c
@@ -1,8 +1,11 @@
 #include <linux/cache.h>
 #include <linux/capability.h>
+#include <linux/cpu.h>
 #include <linux/init.h>
 #include <linux/prctl.h>
 #include <linux/sched.h>
+#include <linux/spinlock.h>
+#include <linux/sysctl.h>
 
 #include <asm/cpu_has_feature.h>
 #include <asm/cputable.h>
@@ -18,6 +21,58 @@
 #define DEXCR_PRCTL_EDITABLE (DEXCR_PRO_SBHE | DEXCR_PRO_IBRTPD | \
 			      DEXCR_PRO_SRAPD | DEXCR_PRO_NPHIE)
 
+/*
+ * Lock to protect system DEXCR override from concurrent updates.
+ * RCU semantics: writers take lock, readers are unlocked.
+ * Writers ensure the memory update is atomic, readers read
+ * atomically.
+ */
+static DEFINE_SPINLOCK(dexcr_sys_enforced_write_lock);
+
+struct mask_override {
+	union {
+		struct {
+			unsigned int mask;
+			unsigned int override;
+		};
+
+		/* Raw access for atomic read/write */
+		unsigned long all;
+	};
+};
+
+static struct mask_override dexcr_sys_enforced;
+
+static int spec_branch_hint_enable = -1;
+
+static void update_userspace_system_dexcr(unsigned int pro_mask, int value)
+{
+	struct mask_override update = { .all = 0 };
+
+	switch (value) {
+	case -1:  /* Clear the mask bit, clear the override bit */
+		break;
+	case 0:  /* Set the mask bit, clear the override bit */
+		update.mask |= pro_mask;
+		break;
+	case 1:  /* Set the mask bit, set the override bit */
+		update.mask |= pro_mask;
+		update.override |= pro_mask;
+		break;
+	}
+
+	spin_lock(&dexcr_sys_enforced_write_lock);
+
+	/* Use the existing values for the non-updated bits */
+	update.mask |= dexcr_sys_enforced.mask & ~pro_mask;
+	update.override |= dexcr_sys_enforced.override & ~pro_mask;
+
+	/* Atomically update system enforced aspects */
+	WRITE_ONCE(dexcr_sys_enforced.all, update.all);
+
+	spin_unlock(&dexcr_sys_enforced_write_lock);
+}
+
 static int __init dexcr_init(void)
 {
 	if (!early_cpu_has_feature(CPU_FTR_ARCH_31))
@@ -25,6 +80,9 @@ static int __init dexcr_init(void)
 
 	mtspr(SPRN_DEXCR, DEFAULT_DEXCR);
 
+	if (early_cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+		update_userspace_system_dexcr(DEXCR_PRO_SBHE, spec_branch_hint_enable);
+
 	return 0;
 }
 early_initcall(dexcr_init);
@@ -52,9 +110,15 @@ unsigned long get_thread_dexcr(struct thread_struct const *t)
 {
 	unsigned long dexcr = DEFAULT_DEXCR;
 
+	/* Atomically read enforced mask & override */
+	struct mask_override enforced = READ_ONCE(dexcr_sys_enforced);
+
 	/* Apply prctl overrides */
 	dexcr = (dexcr & ~t->dexcr_mask) | t->dexcr_override;
 
+	/* Apply system overrides */
+	dexcr = (dexcr & ~enforced.mask) | enforced.override;
+
 	return dexcr;
 }
 
@@ -176,3 +240,64 @@ int dexcr_prctl_set(struct task_struct *task, unsigned long which, unsigned long
 
 	return 0;
 }
+
+#ifdef CONFIG_SYSCTL
+
+static const int min_sysctl_val = -1;
+
+static int sysctl_dexcr_sbhe_handler(struct ctl_table *table, int write,
+				     void *buf, size_t *lenp, loff_t *ppos)
+{
+	int err;
+	int prev = spec_branch_hint_enable;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (!cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+		return -ENODEV;
+
+	err = proc_dointvec_minmax(table, write, buf, lenp, ppos);
+	if (err)
+		return err;
+
+	if (prev != spec_branch_hint_enable && write) {
+		update_userspace_system_dexcr(DEXCR_PRO_SBHE, spec_branch_hint_enable);
+		cpus_read_lock();
+		on_each_cpu(update_dexcr_on_cpu, NULL, 1);
+		cpus_read_unlock();
+	}
+
+	return 0;
+}
+
+static struct ctl_table dexcr_sbhe_ctl_table[] = {
+	{
+		.procname	= "speculative_branch_hint_enable",
+		.data		= &spec_branch_hint_enable,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= sysctl_dexcr_sbhe_handler,
+		.extra1		= (void *)&min_sysctl_val,
+		.extra2		= SYSCTL_ONE,
+	},
+	{}
+};
+
+static struct ctl_table dexcr_sbhe_ctl_root[] = {
+	{
+		.procname	= "kernel",
+		.mode		= 0555,
+		.child		= dexcr_sbhe_ctl_table,
+	},
+	{}
+};
+
+static int __init register_dexcr_aspects_sysctl(void)
+{
+	register_sysctl_table(dexcr_sbhe_ctl_root);
+	return 0;
+}
+device_initcall(register_dexcr_aspects_sysctl);
+
+#endif /* CONFIG_SYSCTL */
-- 
2.38.1