[RFC PATCH 08/13] powerpc/dexcr: Add enforced userspace ROP protection config

[Benjamin Gray]

The DEXCR Non-Privileged Hash Instruction Enable (NPHIE) aspect controls
whether the hashst and hashchk instructions are treated as no-ops by the
CPU.

NPHIE behaviour per ISA 3.1B:

0:	hashst and hashchk instructions are executed as no-ops
	(even when allowed by PCR)

1:	hashst and hashchk instructions are executed normally
	(if allowed by PCR)

Currently this aspect may be set per-process by prctl() or enforced
globally by the hypervisor.

Add a kernel config option PPC_USER_ROP_PROTECT to enforce DEXCR[NPHIE]
globally regardless of prctl() or hypervisor. If set, don't report
NPHIE as editable via prctl(), as the prctl() value can never take
effect.

Signed-off-by: Benjamin Gray <bgray@xxxxxxxxxxxxx>
---
 arch/powerpc/Kconfig        |  5 +++++
 arch/powerpc/kernel/dexcr.c | 15 +++++++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 699df27b0e2f..ba3458d07744 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -434,6 +434,11 @@ config PGTABLE_LEVELS
 	default 2 if !PPC64
 	default 4
 
+config PPC_USER_ROP_PROTECT
+	bool
+	depends on PPC_BOOK3S_64
+	default y
+
 source "arch/powerpc/sysdev/Kconfig"
 source "arch/powerpc/platforms/Kconfig"
 
diff --git a/arch/powerpc/kernel/dexcr.c b/arch/powerpc/kernel/dexcr.c
index 8239bcc92026..394140fc23aa 100644
--- a/arch/powerpc/kernel/dexcr.c
+++ b/arch/powerpc/kernel/dexcr.c
@@ -2,6 +2,7 @@
 #include <linux/capability.h>
 #include <linux/cpu.h>
 #include <linux/init.h>
+#include <linux/kconfig.h>
 #include <linux/prctl.h>
 #include <linux/sched.h>
 #include <linux/spinlock.h>
@@ -18,8 +19,8 @@
 #define DEFAULT_DEXCR	0
 
 /* Allow process configuration of these by default */
-#define DEXCR_PRCTL_EDITABLE (DEXCR_PRO_SBHE | DEXCR_PRO_IBRTPD | \
-			      DEXCR_PRO_SRAPD | DEXCR_PRO_NPHIE)
+static unsigned long dexcr_prctl_editable __ro_after_init =
+	DEXCR_PRO_SBHE | DEXCR_PRO_IBRTPD | DEXCR_PRO_SRAPD | DEXCR_PRO_NPHIE;
 
 /*
  * Lock to protect system DEXCR override from concurrent updates.
@@ -83,6 +84,12 @@ static int __init dexcr_init(void)
 	if (early_cpu_has_feature(CPU_FTR_DEXCR_SBHE))
 		update_userspace_system_dexcr(DEXCR_PRO_SBHE, spec_branch_hint_enable);
 
+	if (early_cpu_has_feature(CPU_FTR_DEXCR_NPHIE) &&
+	    IS_ENABLED(CONFIG_PPC_USER_ROP_PROTECT)) {
+		update_userspace_system_dexcr(DEXCR_PRO_NPHIE, 1);
+		dexcr_prctl_editable &= ~DEXCR_PRO_NPHIE;
+	}
+
 	return 0;
 }
 early_initcall(dexcr_init);
@@ -131,7 +138,7 @@ static int dexcr_aspect_get(struct task_struct *task, unsigned int aspect)
 {
 	int ret = 0;
 
-	if (aspect & DEXCR_PRCTL_EDITABLE)
+	if (aspect & dexcr_prctl_editable)
 		ret |= PR_PPC_DEXCR_PRCTL;
 
 	if (aspect & task->thread.dexcr_mask) {
@@ -174,7 +181,7 @@ int dexcr_prctl_get(struct task_struct *task, unsigned long which)
 
 static int dexcr_aspect_set(struct task_struct *task, unsigned int aspect, unsigned long ctrl)
 {
-	if (!(aspect & DEXCR_PRCTL_EDITABLE))
+	if (!(aspect & dexcr_prctl_editable))
 		return -ENXIO;  /* Aspect is not allowed to be changed by prctl */
 
 	if (aspect & task->thread.dexcr_forced)
-- 
2.38.1