s390/keyboard: avoid off-by-one when using strnlen_user()

strnlen_user() returns the length of the string including terminating 0.
So avoid counting it again and unnecessarily reducing maximum string
size by 1.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/drivers/s390/char/keyboard.c b/drivers/s390/char/keyboard.c
index 01463b052ae7..ef04a9f7a704 100644
--- a/drivers/s390/char/keyboard.c
+++ b/drivers/s390/char/keyboard.c
@@ -433,20 +433,23 @@ do_kdgkb_ioctl(struct kbd_data *kbd, struct kbsentry __user *u_kbs,
         case KDSKBSENT:
                 if (!perm)
                         return -EPERM;
-                len = strnlen_user(u_kbs->kb_string,
-                                   sizeof(u_kbs->kb_string) - 1);
+                len = strnlen_user(u_kbs->kb_string, sizeof(u_kbs->kb_string));
                 if (!len)
                         return -EFAULT;
-                if (len > sizeof(u_kbs->kb_string) - 1)
+                if (len > sizeof(u_kbs->kb_string))
                         return -EINVAL;
-                p = kmalloc(len + 1, GFP_KERNEL);
+                p = kmalloc(len, GFP_KERNEL);
                 if (!p)
                         return -ENOMEM;
                 if (copy_from_user(p, u_kbs->kb_string, len)) {
                         kfree(p);
                         return -EFAULT;
                 }
-                p[len] = 0;
+                /*
+                 * Make sure the string is terminated by 0. User could have
+                 * modified it between us running strnlen_user() and copying it.
+                 */
+                p[len - 1] = 0;
                 kfree(kbd->func_table[kb_func]);
                 kbd->func_table[kb_func] = p;
                 break;