# \[PATCH net v3 2/2\] net: tcp: block standard payload injection into devmem skbs

- From: Mina Almasry \<hidden\>
- Date: 2026-08-11 19:54:07 
- List: netdev
- Page: https://ratatoskr.run/netdev/2026/08/17398682
- Thread: https://ratatoskr.run/netdev/2026/08/17398683/t.md

```text
Protect tcp_sendmsg_locked() from mistakenly appending non-zerocopy
page fragments to unreadable devmem skbs. Create a new segment instead.

Fixes: bd61848900bff ("net: devmem: Implement TX path")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Bobby Eshleman <redacted>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Signed-off-by: Mina Almasry <redacted>
---
 net/ipv4/tcp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 455441f1b6949..186a36c698798 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1278,6 +1278,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 		if (copy > msg_data_left(msg))
 			copy = msg_data_left(msg);
 
+		if (zc != MSG_ZEROCOPY && unlikely(!skb_frags_readable(skb))) {
+			tcp_mark_push(tp, skb);
+			goto new_segment;
+		}
+
 		if (zc == 0) {
 			bool merge = true;
 			int i = skb_shinfo(skb)->nr_frags;
-- 
2.55.0.679.g6767b8d81c-goog

```