[PATCH v1 5/6] powerpc/32s: prepare prevent_user_access() for user_access_end()

[Christophe Leroy <christophe.leroy@c-s.fr>]

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 case named KUAP_SELF. In this case only,
the addr and end are retrieved from current->kuap.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
 arch/powerpc/include/asm/book3s/32/kup.h | 25 ++++++++++++++++++------
 arch/powerpc/include/asm/kup.h           |  1 +
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 3c1798e56b55..a99fc3428ac9 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -117,6 +117,7 @@ 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 +128,27 @@ 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_W))
-		return;
 
-	addr = (__force u32)to;
+	if (dir == KUAP_SELF) {
+		u32 kuap = current->thread.kuap;
 
-	if (unlikely(addr >= TASK_SIZE || !size))
-		return;
+		if (unlikely(!kuap))
+			return;
+
+		addr = kuap & 0xf0000000;
+		end = kuap << 28;
+	} else {
+		if (!(dir & KUAP_W))
+			return;
+
+		addr = (__force u32)to;
+
+		if (unlikely(addr >= TASK_SIZE || !size))
+			return;
+
+		end = min(addr + size, TASK_SIZE);
+	}
 
-	end = min(addr + size, TASK_SIZE);
 	current->thread.kuap = 0;
 	kuap_update_sr(mfsrin(addr) | SR_KS, addr, end);	/* set Ks */
 }
diff --git a/arch/powerpc/include/asm/kup.h b/arch/powerpc/include/asm/kup.h
index 49fd977a498e..0676df6f3f86 100644
--- a/arch/powerpc/include/asm/kup.h
+++ b/arch/powerpc/include/asm/kup.h
@@ -5,6 +5,7 @@
 #define KUAP_R		1
 #define KUAP_W		2
 #define KUAP_RW		(KUAP_R | KUAP_W)
+#define KUAP_SELF	4
 
 #ifdef CONFIG_PPC64
 #include <asm/book3s/64/kup-radix.h>
-- 
2.25.0