ima: fix ima_d_path() possible race with rename

On failure to return a pathname from ima_d_path(), a pointer to
dname is returned, which is subsequently used in the IMA measurement
list, the IMA audit records, and other audit logging.  Saving the
pointer to dname for later use has the potential to race with rename.

Intead of returning a pointer to dname on failure, this patch returns
a pointer to a copy of the filename.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org

security/integrity/ima/ima_api.c

@@ -318,7 +318,17 @@ void ima_audit_measurement(struct integrity_iint_cache *iint,
 	iint->flags |= IMA_AUDITED;
 }
 
-const char *ima_d_path(const struct path *path, char **pathbuf)
+/*
+ * ima_d_path - return a pointer to the full pathname
+ *
+ * Attempt to return a pointer to the full pathname for use in the
+ * IMA measurement list, IMA audit records, and auditing logs.
+ *
+ * On failure, return a pointer to a copy of the filename, not dname.
+ * Returning a pointer to dname, could result in using the pointer
+ * after the memory has been freed.
+ */
+const char *ima_d_path(const struct path *path, char **pathbuf, char *namebuf)
 {
 	char *pathname = NULL;
 
@@ -331,5 +341,11 @@ const char *ima_d_path(const struct path *path, char **pathbuf)
 			pathname = NULL;
 		}
 	}
-	return pathname ?: (const char *)path->dentry->d_name.name;
+
+	if (!pathname) {
+		strlcpy(namebuf, path->dentry->d_name.name, NAME_MAX);
+		pathname = namebuf;
+	}
+
+	return pathname;
 }