[RFC PATCH 6/6] powerpc/dexcr: Add enforced userspace ROP protection config

[Benjamin Gray <bgray@linux.ibm.com>]

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_ENFORCE_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@linux.ibm.com>
---
 arch/powerpc/Kconfig        | 5 +++++
 arch/powerpc/kernel/dexcr.c | 7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 3aaadfd2c8eb..4851cb463dc0 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -473,6 +473,11 @@ config PGTABLE_LEVELS
 	default 2 if !PPC64
 	default 4
 
+config PPC_USER_ENFORCE_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 e790f76787db..01d8fa28ca17 100644
--- a/arch/powerpc/kernel/dexcr.c
+++ b/arch/powerpc/kernel/dexcr.c
@@ -1,6 +1,8 @@
+#include <linux/cache.h>
 #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/sysctl.h>
@@ -14,7 +16,7 @@
 #define DEXCR_PRCTL_EDITABLE ( \
 	DEXCR_PR_IBRTPD | \
 	DEXCR_PR_SRAPD | \
-	DEXCR_PR_NPHIE)
+	(!IS_ENABLED(CONFIG_PPC_USER_ENFORCE_ROP_PROTECT) ? DEXCR_PR_NPHIE : 0))
 
 static unsigned long dexcr_supported __ro_after_init = 0;
 
@@ -45,6 +47,9 @@ unsigned long get_thread_dexcr(struct thread_struct const *thread)
 {
 	unsigned long dexcr = thread->dexcr_enabled;
 
+	if (IS_ENABLED(CONFIG_PPC_USER_ENFORCE_ROP_PROTECT))
+		dexcr |= DEXCR_PR_NPHIE;
+
 	/* 
 	 * spec_branch_hint_enable may be written to concurrently via sysctl.
 	 * The sysctl handler is careful to use WRITE_ONCE, so we avoid
-- 
2.41.0