ptrace, seccomp: tweak get_metadata behavior slightly

Previously if users passed a small size for the input structure size, they
would get get odd behavior. It doesn't make sense to pass a structure
smaller than at least filter_off size, so let's just give -EINVAL in this
case.

This changes userspace visible behavior, but was only introduced in commit
26500475 ("ptrace, seccomp: add support for retrieving seccomp
metadata") in 4.16-rc2, so should be safe to change if merged before then.
Reported-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/seccomp.c

@@ -1076,14 +1076,16 @@ long seccomp_get_metadata(struct task_struct *task,
 
 	size = min_t(unsigned long, size, sizeof(kmd));
 
-	if (copy_from_user(&kmd, data, size))
+	if (size < sizeof(kmd.filter_off))
+		return -EINVAL;
+
+	if (copy_from_user(&kmd.filter_off, data, sizeof(kmd.filter_off)))
 		return -EFAULT;
 
 	filter = get_nth_filter(task, kmd.filter_off);
 	if (IS_ERR(filter))
 		return PTR_ERR(filter);
 
-	memset(&kmd, 0, sizeof(kmd));
 	if (filter->log)
 		kmd.flags |= SECCOMP_FILTER_FLAG_LOG;