cifs: potential memory leaks when parsing mnt opts

For example, when mount opt is redundently specified
(e.g., "user=A,user=B,user=C"), kernel kept allocating new key/val
with kstrdup() and overwrite previous ptr (to be freed).

Althouhg mount.cifs in userspace performs a bit of sanitization
(e.g., forcing one user option), current implementation is not
robust. Other options such as iocharset and domainanme are similarly
vulnerable.
Signed-off-by: Taesoo Kim <tsgatesv@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/connect.c

@@ -1599,6 +1599,8 @@ cifs_parse_mount_options(const char *mountdata, const char *devname,
 				pr_warn("CIFS: username too long\n");
 				goto cifs_parse_mount_err;
 			}
+
+			kfree(vol->username);
 			vol->username = kstrdup(string, GFP_KERNEL);
 			if (!vol->username)
 				goto cifs_parse_mount_err;
@@ -1700,6 +1702,7 @@ cifs_parse_mount_options(const char *mountdata, const char *devname,
 				goto cifs_parse_mount_err;
 			}
 
+			kfree(vol->domainname);
 			vol->domainname = kstrdup(string, GFP_KERNEL);
 			if (!vol->domainname) {
 				pr_warn("CIFS: no memory for domainname\n");
@@ -1731,6 +1734,7 @@ cifs_parse_mount_options(const char *mountdata, const char *devname,
 			}
 
 			 if (strncasecmp(string, "default", 7) != 0) {
+				kfree(vol->iocharset);
 				vol->iocharset = kstrdup(string,
 							 GFP_KERNEL);
 				if (!vol->iocharset) {