powerpc/32s: Prepare prevent_user_access() for user_access_end()

In preparation of implementing user_access_begin and friends
on powerpc, the book3s/32 version of prevent_user_access() need
to be prepared for user_access_end().

user_access_end() doesn't provide the address and size which
were passed to user_access_begin(), required by prevent_user_access()
to know which segment to modify.

The list of segments which where unprotected by allow_user_access()
are available in current->kuap. But we don't want prevent_user_access()
to read this all the time, especially everytime it is 0 (for instance
because the access was not a write access).

Implement a special direction named KUAP_CURRENT. In this case only,
the addr and end are retrieved from current->kuap.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/55bcc1f25d8200892a31f67a0b024ff3b816c3cc.1579866752.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/book3s/32/kup.h

@@ -108,6 +108,8 @@ static __always_inline void allow_user_access(void __user *to, const void __user
 	u32 addr, end;
 
 	BUILD_BUG_ON(!__builtin_constant_p(dir));
+	BUILD_BUG_ON(dir == KUAP_CURRENT);
+
 	if (!(dir & KUAP_WRITE))
 		return;
 
@@ -117,6 +119,7 @@ static __always_inline void allow_user_access(void __user *to, const void __user
 		return;
 
 	end = min(addr + size, TASK_SIZE);
+
 	current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
 	kuap_update_sr(mfsrin(addr) & ~SR_KS, addr, end);	/* Clear Ks */
 }
@@ -127,15 +130,25 @@ static __always_inline void prevent_user_access(void __user *to, const void __us
 	u32 addr, end;
 
 	BUILD_BUG_ON(!__builtin_constant_p(dir));
-	if (!(dir & KUAP_WRITE))
-		return;
 
-	addr = (__force u32)to;
+	if (dir == KUAP_CURRENT) {
+		u32 kuap = current->thread.kuap;
 
-	if (unlikely(addr >= TASK_SIZE || !size))
+		if (unlikely(!kuap))
+			return;
+
+		addr = kuap & 0xf0000000;
+		end = kuap << 28;
+	} else if (dir & KUAP_WRITE) {
+		addr = (__force u32)to;
+		end = min(addr + size, TASK_SIZE);
+
+		if (unlikely(addr >= TASK_SIZE || !size))
+			return;
+	} else {
 		return;
+	}
 
-	end = min(addr + size, TASK_SIZE);
 	current->thread.kuap = 0;
 	kuap_update_sr(mfsrin(addr) | SR_KS, addr, end);	/* set Ks */
 }

arch/powerpc/include/asm/book3s/64/kup-radix.h

@@ -86,8 +86,10 @@ static __always_inline void allow_user_access(void __user *to, const void __user
 		set_kuap(AMR_KUAP_BLOCK_WRITE);
 	else if (dir == KUAP_WRITE)
 		set_kuap(AMR_KUAP_BLOCK_READ);
-	else
+	else if (dir == KUAP_READ_WRITE)
 		set_kuap(0);
+	else
+		BUILD_BUG();
 }
 
 static inline void prevent_user_access(void __user *to, const void __user *from,

arch/powerpc/include/asm/kup.h

@@ -5,6 +5,12 @@
 #define KUAP_READ	1
 #define KUAP_WRITE	2
 #define KUAP_READ_WRITE	(KUAP_READ | KUAP_WRITE)
+/*
+ * For prevent_user_access() only.
+ * Use the current saved situation instead of the to/from/size params.
+ * Used on book3s/32
+ */
+#define KUAP_CURRENT	4
 
 #ifdef CONFIG_PPC64
 #include <asm/book3s/64/kup-radix.h>
@@ -88,6 +94,11 @@ static inline void prevent_read_write_user(void __user *to, const void __user *f
 	prevent_user_access(to, from, size, KUAP_READ_WRITE);
 }
 
+static inline void prevent_current_access_user(void)
+{
+	prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT);
+}
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* _ASM_POWERPC_KUAP_H_ */