dlm: check the write size from user

Return EINVAL from write if the size is larger than
allowed.  Do this before allocating kernel memory for
the bogus size, which could lead to OOM.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Tested-by: Jana Saout <jana@saout.de>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/user.c

@@ -503,11 +503,11 @@ static ssize_t device_write(struct file *file, const char __user *buf,
 #endif
 		return -EINVAL;
 
-#ifdef CONFIG_COMPAT
-	if (count > sizeof(struct dlm_write_request32) + DLM_RESNAME_MAXLEN)
-#else
+	/*
+	 * can't compare against COMPAT/dlm_write_request32 because
+	 * we don't yet know if is64bit is zero
+	 */
 	if (count > sizeof(struct dlm_write_request) + DLM_RESNAME_MAXLEN)
-#endif
 		return -EINVAL;
 
 	kbuf = kzalloc(count + 1, GFP_NOFS);