linux-kernel - [PATCH v2 07/12] powerpc/32s: Allow disabling KUEP at boot time

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Date:   Thu,  3 Jun 2021 08:41:42 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 07/12] powerpc/32s: Allow disabling KUEP at boot time

PPC64 uses MMU features to enable/disable KUEP at boot time.
But feature fixups are applied way too early on PPC32.

Now that all KUEP related actions are in C following the
conversion of KUEP initial setup and context switch in C,
static branches can be used to enable/disable KUEP.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/include/asm/book3s/32/kup.h |  6 +++++-
 arch/powerpc/mm/book3s32/kuep.c          | 11 +++++++----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 618ffc8e4ee9..2854d970dabe 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -7,6 +7,10 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/jump_label.h>
+
+extern struct static_key_false disable_kuep_key;
+
 static __always_inline bool kuap_is_disabled(void)
 {
 	return !IS_ENABLED(CONFIG_PPC_KUAP);
@@ -14,7 +18,7 @@ static __always_inline bool kuap_is_disabled(void)
 
 static __always_inline bool kuep_is_disabled(void)
 {
-	return !IS_ENABLED(CONFIG_PPC_KUEP);
+	return !IS_ENABLED(CONFIG_PPC_KUEP) || static_branch_unlikely(&disable_kuep_key);
 }
 
 static inline void kuep_lock(void)
diff --git a/arch/powerpc/mm/book3s32/kuep.c b/arch/powerpc/mm/book3s32/kuep.c
index 3147e2edcf63..3f6eb6e23fca 100644
--- a/arch/powerpc/mm/book3s32/kuep.c
+++ b/arch/powerpc/mm/book3s32/kuep.c
@@ -3,15 +3,18 @@
 #include <asm/kup.h>
 #include <asm/smp.h>
 
+struct static_key_false disable_kuep_key;
+
 void __init setup_kuep(bool disabled)
 {
-	kuep_lock();
+	if (!disabled)
+		kuep_lock();
 
 	if (smp_processor_id() != boot_cpuid)
 		return;
 
-	pr_info("Activating Kernel Userspace Execution Prevention\n");
-
 	if (disabled)
-		pr_warn("KUEP cannot be disabled yet on 6xx when compiled in\n");
+		static_branch_enable(&disable_kuep_key);
+	else
+		pr_info("Activating Kernel Userspace Execution Prevention\n");
 }
-- 
2.25.0