tcp: devmem: don't write truncated dmabuf CMSGs to userspace

Currently, we report -ETOOSMALL (err) only on the first iteration
(!sent). When we get put_cmsg error after a bunch of successful
put_cmsg calls, we don't signal the error at all. This might be
confusing on the userspace side which will see truncated CMSGs
but no MSG_CTRUNC signal.

Consider the following case:
- sizeof(struct cmsghdr) = 16
- sizeof(struct dmabuf_cmsg) = 24
- total cmsg size (CMSG_LEN) = 40 (16+24)

When calling recvmsg with msg_controllen=60, the userspace
will receive two(!) dmabuf_cmsg(s), the first one will
be a valid one and the second one will be silently truncated. There is no
easy way to discover the truncation besides doing something like
"cm->cmsg_len != CMSG_LEN(sizeof(dmabuf_cmsg))".

Introduce new put_devmem_cmsg wrapper that reports an error instead
of doing the truncation. Mina suggests that it's the intended way
this API should work.

Note that we might now report MSG_CTRUNC when the users (incorrectly)
call us with msg_control == NULL.

Fixes: 8f0b3cc ("tcp: RX path for devmem TCP")
Reviewed-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250224174401.3582695-1-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/socket.h

@@ -392,6 +392,8 @@ struct ucred {
 
 extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
 extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
+extern int put_cmsg_notrunc(struct msghdr *msg, int level, int type, int len,
+			    void *data);
 
 struct timespec64;
 struct __kernel_timespec;

net/core/scm.c

@@ -282,6 +282,16 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 }
 EXPORT_SYMBOL(put_cmsg);
 
+int put_cmsg_notrunc(struct msghdr *msg, int level, int type, int len,
+		     void *data)
+{
+	/* Don't produce truncated CMSGs */
+	if (!msg->msg_control || msg->msg_controllen < CMSG_LEN(len))
+		return -ETOOSMALL;
+
+	return put_cmsg(msg, level, type, len, data);
+}
+
 void put_cmsg_scm_timestamping64(struct msghdr *msg, struct scm_timestamping_internal *tss_internal)
 {
 	struct scm_timestamping64 tss;

net/ipv4/tcp.c

@@ -2438,14 +2438,12 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 			 */
 			memset(&dmabuf_cmsg, 0, sizeof(dmabuf_cmsg));
 			dmabuf_cmsg.frag_size = copy;
-			err = put_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
-				       sizeof(dmabuf_cmsg), &dmabuf_cmsg);
-			if (err || msg->msg_flags & MSG_CTRUNC) {
-				msg->msg_flags &= ~MSG_CTRUNC;
-				if (!err)
-					err = -ETOOSMALL;
+			err = put_cmsg_notrunc(msg, SOL_SOCKET,
+					       SO_DEVMEM_LINEAR,
+					       sizeof(dmabuf_cmsg),
+					       &dmabuf_cmsg);
+			if (err)
 				goto out;
-			}
 
 			sent += copy;
 
@@ -2499,16 +2497,12 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 				offset += copy;
 				remaining_len -= copy;
 
-				err = put_cmsg(msg, SOL_SOCKET,
-					       SO_DEVMEM_DMABUF,
-					       sizeof(dmabuf_cmsg),
-					       &dmabuf_cmsg);
-				if (err || msg->msg_flags & MSG_CTRUNC) {
-					msg->msg_flags &= ~MSG_CTRUNC;
-					if (!err)
-						err = -ETOOSMALL;
+				err = put_cmsg_notrunc(msg, SOL_SOCKET,
+						       SO_DEVMEM_DMABUF,
+						       sizeof(dmabuf_cmsg),
+						       &dmabuf_cmsg);
+				if (err)
 					goto out;
-				}
 
 				atomic_long_inc(&niov->pp_ref_count);
 				tcp_xa_pool.netmems[tcp_xa_pool.idx++] = skb_frag_netmem(frag);