af_unix: Fix msg_controllen test in scm_pidfd_recv() for MSG_CMSG_COMPAT.

Heiko Carstens reported that SCM_PIDFD does not work with MSG_CMSG_COMPAT
because scm_pidfd_recv() always checks msg_controllen against sizeof(struct
cmsghdr).

We need to use sizeof(struct compat_cmsghdr) for the compat case.

Fixes: 5e2ff670 ("scm: add SO_PASSPIDFD and SCM_PIDFD")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Closes: https://lore.kernel.org/netdev/20230901200517.8742-A-hca@linux.ibm.com/Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/scm.h

@@ -9,6 +9,7 @@
 #include <linux/pid.h>
 #include <linux/nsproxy.h>
 #include <linux/sched/signal.h>
+#include <net/compat.h>
 
 /* Well, we should have at least one descriptor open
  * to accept passed FDs 8)
@@ -123,14 +124,17 @@ static inline bool scm_has_secdata(struct socket *sock)
 static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm)
 {
 	struct file *pidfd_file = NULL;
-	int pidfd;
+	int len, pidfd;
 
-	/*
-	 * put_cmsg() doesn't return an error if CMSG is truncated,
+	/* put_cmsg() doesn't return an error if CMSG is truncated,
 	 * that's why we need to opencode these checks here.
 	 */
-	if ((msg->msg_controllen <= sizeof(struct cmsghdr)) ||
-	    (msg->msg_controllen - sizeof(struct cmsghdr)) < sizeof(int)) {
+	if (msg->msg_flags & MSG_CMSG_COMPAT)
+		len = sizeof(struct compat_cmsghdr) + sizeof(int);
+	else
+		len = sizeof(struct cmsghdr) + sizeof(int);
+
+	if (msg->msg_controllen < len) {
 		msg->msg_flags |= MSG_CTRUNC;
 		return;
 	}