[PATCH net v2] tcp: reject devmem tx with fastopen and repair

[Kaifeng Wang <kaifengw@google.com>]

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 the caller's memory
   at those addresses 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 inside the Fast Open and TCP repair branches when a devmem
binding is present. Checking inside the tp->repair branch also covers
the case where repair mode is enabled while sk_stream_wait_connect()
drops the socket lock.

Additionally, drop the !sockc_err gate from the devmem validation check
and return sockc_err if set. Otherwise, if a valid SCM_DEVMEM_DMABUF is
followed by a failing cmsg, sockc.dmabuf_id remains set while binding is
not acquired, allowing the Fast Open and TCP_RECV_QUEUE repair paths to
proceed and return before sockc_err is checked.

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")
Reviewed-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Kaifeng Wang <kaifengw@google.com>
---
v1 -> v2:
- Move the devmem rejection checks into the Fast Open and TCP repair
  branches directly, also covering the case where tp->repair is enabled
  while sk_stream_wait_connect() drops the socket lock.
- Drop the !sockc_err gate from the devmem validation check and return
  sockc_err if set, preventing a valid SCM_DEVMEM_DMABUF followed by a
  failing cmsg from bypassing the check.

 net/ipv4/tcp.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 87ef6d5cbfeb..2792923f0748 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1169,14 +1169,18 @@ 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)) {
-		err = -EINVAL;
+	if (sockc.dmabuf_id && (zc != MSG_ZEROCOPY || !binding)) {
+		err = sockc_err ?: -EINVAL;
 		goto out_err;
 	}
 
 	if (unlikely(flags & MSG_FASTOPEN ||
 		     inet_test_bit(DEFER_CONNECT, sk)) &&
 	    !tp->repair) {
+		if (binding) {
+			err = -EINVAL;
+			goto out_err;
+		}
 		err = tcp_sendmsg_fastopen(sk, msg, &copied_syn, size, uarg);
 		if (err == -EINPROGRESS && copied_syn > 0)
 			goto out;
@@ -1200,6 +1204,10 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 	}
 
 	if (unlikely(tp->repair)) {
+		if (binding) {
+			err = -EINVAL;
+			goto out_err;
+		}
 		if (tp->repair_queue == TCP_RECV_QUEUE) {
 			copied = tcp_send_rcvq(sk, msg, size);
 			goto out_nopush;
-- 
2.56.0.385.gd3acb90ef8-goog