powerpc/kuap: Use ASM feature fixups instead of static branches

To avoid a useless nop on top of every uaccess enable/disable and
make life easier for objtool, replace static branches by ASM feature
fixups that will nop KUAP enabling instructions out in the unlikely
case KUAP is disabled at boottime.

Leave it as is on book3s/64 for now, it will be handled later when
objtool is activated on PPC64.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/671948788024fd890ec4ed175bc332dab8664ea5.1689091022.git.christophe.leroy@csgroup.eu

1 files changed, 37 insertions, 9 deletions

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 931d200afe56..4e14a5427a63 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -27,6 +27,34 @@ static __always_inline void kuap_unlock_one(unsigned long addr)
 	isync();	/* Context sync required after mtsr() */
 }
 
+static __always_inline void uaccess_begin_32s(unsigned long addr)
+{
+	unsigned long tmp;
+
+	asm volatile(ASM_MMU_FTR_IFSET(
+		"mfsrin %0, %1;"
+		"rlwinm %0, %0, 0, %2;"
+		"mtsrin %0, %1;"
+		"isync", "", %3)
+		: "=&r"(tmp)
+		: "r"(addr), "i"(~SR_KS), "i"(MMU_FTR_KUAP)
+		: "memory");
+}
+
+static __always_inline void uaccess_end_32s(unsigned long addr)
+{
+	unsigned long tmp;
+
+	asm volatile(ASM_MMU_FTR_IFSET(
+		"mfsrin %0, %1;"
+		"oris %0, %0, %2;"
+		"mtsrin %0, %1;"
+		"isync", "", %3)
+		: "=&r"(tmp)
+		: "r"(addr), "i"(SR_KS >> 16), "i"(MMU_FTR_KUAP)
+		: "memory");
+}
+
 static __always_inline void __kuap_save_and_lock(struct pt_regs *regs)
 {
 	unsigned long kuap = current->thread.kuap;
@@ -69,8 +97,8 @@ static __always_inline unsigned long __kuap_get_and_assert_locked(void)
 }
 #define __kuap_get_and_assert_locked __kuap_get_and_assert_locked
 
-static __always_inline void __allow_user_access(void __user *to, const void __user *from,
-						u32 size, unsigned long dir)
+static __always_inline void allow_user_access(void __user *to, const void __user *from,
+					      u32 size, unsigned long dir)
 {
 	BUILD_BUG_ON(!__builtin_constant_p(dir));
 
@@ -78,10 +106,10 @@ static __always_inline void __allow_user_access(void __user *to, const void __us
 		return;
 
 	current->thread.kuap = (__force u32)to;
-	kuap_unlock_one((__force u32)to);
+	uaccess_begin_32s((__force u32)to);
 }
 
-static __always_inline void __prevent_user_access(unsigned long dir)
+static __always_inline void prevent_user_access(unsigned long dir)
 {
 	u32 kuap = current->thread.kuap;
 
@@ -91,26 +119,26 @@ static __always_inline void __prevent_user_access(unsigned long dir)
 		return;
 
 	current->thread.kuap = KUAP_NONE;
-	kuap_lock_one(kuap);
+	uaccess_end_32s(kuap);
 }
 
-static __always_inline unsigned long __prevent_user_access_return(void)
+static __always_inline unsigned long prevent_user_access_return(void)
 {
 	unsigned long flags = current->thread.kuap;
 
 	if (flags != KUAP_NONE) {
 		current->thread.kuap = KUAP_NONE;
-		kuap_lock_one(flags);
+		uaccess_end_32s(flags);
 	}
 
 	return flags;
 }
 
-static __always_inline void __restore_user_access(unsigned long flags)
+static __always_inline void restore_user_access(unsigned long flags)
 {
 	if (flags != KUAP_NONE) {
 		current->thread.kuap = flags;
-		kuap_unlock_one(flags);
+		uaccess_begin_32s(flags);
 	}
 }