[PATCH 3.2 101/152] asm-generic: make copy_from_user() zero the destination properly

[Ben Hutchings]

3.2.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 2545e5da080b4839dd859e3b09343a884f6ab0e3 upstream.

... in all cases, including the failing access_ok()

Note that some architectures using asm-generic/uaccess.h have
__copy_from_user() not zeroing the tail on failure halfway
through.  This variant works either way.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/asm-generic/uaccess.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- a/include/asm-generic/uaccess.h
+++ b/include/asm-generic/uaccess.h
@@ -243,11 +243,13 @@ extern int __get_user_bad(void) __attrib
 static inline long copy_from_user(void *to,
 		const void __user * from, unsigned long n)
 {
+	unsigned long res = n;
 	might_sleep();
-	if (access_ok(VERIFY_READ, from, n))
-		return __copy_from_user(to, from, n);
-	else
-		return n;
+	if (likely(access_ok(VERIFY_READ, from, n)))
+		res = __copy_from_user(to, from, n);
+	if (unlikely(res))
+		memset(to + (n - res), 0, res);
+	return res;
 }
 
 static inline long copy_to_user(void __user *to,