usercopy: add testcases to check zeroing on failure

During usercopy the destination buffer will be zeroed if copy_from_user()
or get_user() fails. This patch adds testcases for it. The destination
buffer is set with non-zero value before illegal copy_from_user() or
get_user() is executed and the buffer is compared to zero after usercopy
is done.

Signed-off-by: Hoeun Ryu <hoeun.ryu@gmail.com>
[kees: clarified commit log, dropped second kmalloc]
Signed-off-by: Kees Cook <keescook@chromium.org>

lib/test_user_copy.c

@@ -69,20 +69,30 @@ static int __init test_user_copy_init(void)
 		    "legitimate put_user failed");
 
 	/* Invalid usage: none of these should succeed. */
+	memset(kmem, 0x5a, PAGE_SIZE);
+	memset(kmem + PAGE_SIZE, 0, PAGE_SIZE);
 	ret |= test(!copy_from_user(kmem, (char __user *)(kmem + PAGE_SIZE),
 				    PAGE_SIZE),
 		    "illegal all-kernel copy_from_user passed");
+	ret |= test(memcmp(kmem + PAGE_SIZE, kmem, PAGE_SIZE),
+		    "zeroing failure for illegal all-kernel copy_from_user");
+	memset(bad_usermem, 0x5A, PAGE_SIZE);
 	ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
 				    PAGE_SIZE),
 		    "illegal reversed copy_from_user passed");
+	ret |= test(memcmp(kmem + PAGE_SIZE, bad_usermem, PAGE_SIZE),
+		    "zeroing failure for illegal reversed copy_from_user");
 	ret |= test(!copy_to_user((char __user *)kmem, kmem + PAGE_SIZE,
 				  PAGE_SIZE),
 		    "illegal all-kernel copy_to_user passed");
 	ret |= test(!copy_to_user((char __user *)kmem, bad_usermem,
 				  PAGE_SIZE),
 		    "illegal reversed copy_to_user passed");
+	memset(kmem, 0x5a, PAGE_SIZE);
 	ret |= test(!get_user(value, (unsigned long __user *)kmem),
 		    "illegal get_user passed");
+	ret |= test(memcmp(kmem + PAGE_SIZE, kmem, sizeof(value)),
+		    "zeroing failure for illegal get_user");
 	ret |= test(!put_user(value, (unsigned long __user *)kmem),
 		    "illegal put_user passed");