[PATCH net v4 2/2] net: tcp: block mixing readable and unreadable frags

Protect tcp_sendmsg_locked() from mistakenly mixing readable and
unreadable page fragments in the same SKB.

Check that the devmem binding matches the existing SKB's readability.
If a mismatch is detected, avoid collapsing and create a new segment.

Fixes: bd61848900bff ("net: devmem: Implement TX path")
Suggested-by: Eric Dumazet <edumazet@google.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Bobby Eshleman <redacted>
Signed-off-by: Mina Almasry <redacted>
---
v4:
- Adopt Eric's suggestion to check 'binding' against SKB readability
  alongside tcp_skb_can_collapse_to().
- Drop pre-existing Reviewed-by tags due to significant implementation changes.
v3: https://lore.kernel.org/r/20260811195405.3979177-2-almasrymina@google.com (local)
v2: https://lore.kernel.org/r/20260810180956.2348280-2-almasrymina@google.com (local)
v1: https://lore.kernel.org/r/20260801125308.1342897-3-almasrymina@google.com (local)
---
 net/ipv4/tcp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 455441f1b6949..b4237d0e994d6 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -1240,7 +1240,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 		trace_tcp_sendmsg_locked(sk, msg, skb, size_goal);
 
-		if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
+		if (copy <= 0 || !tcp_skb_can_collapse_to(skb) ||
+		    unlikely(skb_frags_readable(skb) != !binding)) {
 			bool first_skb;
 
 new_segment:

-- 
2.55.0.691.gc56d675ccc-goog