[PATCH net] tcp: reject devmem tx with fastopen and repair

tcp_sendmsg_locked() enforces that devmem TX can only proceed if the
zero-copy path is active and a valid dmabuf binding exists. However,
subsequent branches in tcp_sendmsg_locked() can still intercept the
message before it reaches the devmem zero-copy loop:

1. TCP Fast Open (MSG_FASTOPEN or DEFER_CONNECT):
   If TCP_FASTOPEN_CONNECT is set, the socket may have a valid dst with
   NETIF_F_SG (so zc == MSG_ZEROCOPY and binding is present), but
   tcp_sendmsg_fastopen() -> tcp_send_syn_data() will use
   copy_page_from_iter() to byte-copy from the iterator. Since iov_base
   represents dma-buf offsets rather than user virtual addresses, this
   misinterprets offsets as user pointers and copies arbitrary user memory
   into the SYN packet.

2. TCP repair mode:
   If tp->repair is enabled with TCP_RECV_QUEUE, tcp_send_rcvq() similarly
   calls skb_copy_datagram_from_iter(), byte-copying from the iterator.

Neither path supports or makes sense for devmem transmission. Reject devmem
sends if Fast Open or repair mode is active.

This pre-existing issue was identified by Sashiko AI review on commit
125755776bc6 ("tcp: reject non zerocopy devmem tx") and has not been
hit in production.

Fixes: 125755776bc6 ("tcp: reject non zerocopy devmem tx")
Fixes: bd61848900bf ("net: devmem: Implement TX path")
Signed-off-by: Kaifeng Wang <redacted>
---
 net/ipv4/tcp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 87ef6d5cbfeb..225e758c194a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -1169,7 +1169,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			zc = MSG_SPLICE_PAGES;
 	}
 
-	if (!sockc_err && sockc.dmabuf_id && (zc != MSG_ZEROCOPY || !binding)) {
+	if (!sockc_err && sockc.dmabuf_id &&
+	    (zc != MSG_ZEROCOPY || !binding || tp->repair ||
+	     (flags & MSG_FASTOPEN) || inet_test_bit(DEFER_CONNECT, sk))) {
 		err = -EINVAL;
 		goto out_err;
 	}

-- 
2.56.0.rc1.315.gc6ed9934b7-goog