[PATCH net v2 1/1] tcp: reject non zerocopy devmem tx

Devmem tcp tx doesn't work without zero-copy, however it's not currently
enforced if NETIF_F_SG isn't present. In this case, tcp_sendmsg_locked()
will try the copy path and try to copy data from an iovec which consists
of offsets into the dma-buf and would normally fail. Moreover,
d9c56501c72fd ("net: tcp: block mixing readable and unreadable frags")
relies on that and assumes that the devmem binding is present IFF we're
using the zero-copy path, which can be used to mix net-iov and pages in
a single skb, and break invariants. Let's reject devmem tx without
zero-copy.

Note, the parameter check the patch is modifying is too loose, we can
create an io_uring request with dmabuf_id and all ZC flags, but which
won't have the binding. We replace it with stricter validation.

Fixes: bd61848900bff ("net: devmem: Implement TX path")
Fixes: d9c56501c72fd ("net: tcp: block mixing readable and unreadable frags")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---

v2: combine with post ubuf assignment dmabuf_id checks

 net/ipv4/tcp.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b4237d0e994d..10f58281be17 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -1169,8 +1169,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			zc = MSG_SPLICE_PAGES;
 	}
 
-	if (!sockc_err && sockc.dmabuf_id &&
-	    (!(flags & MSG_ZEROCOPY) || !sock_flag(sk, SOCK_ZEROCOPY))) {
+	if (!sockc_err && sockc.dmabuf_id && (zc != MSG_ZEROCOPY || !binding)) {
 		err = -EINVAL;
 		goto out_err;
 	}

-- 
2.54.0

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni [off-list ref]:

On Fri,  4 Sep 2026 14:43:07 +0100 you wrote:

Devmem tcp tx doesn't work without zero-copy, however it's not currently
enforced if NETIF_F_SG isn't present. In this case, tcp_sendmsg_locked()
will try the copy path and try to copy data from an iovec which consists
of offsets into the dma-buf and would normally fail. Moreover,
d9c56501c72fd ("net: tcp: block mixing readable and unreadable frags")
relies on that and assumes that the devmem binding is present IFF we're
using the zero-copy path, which can be used to mix net-iov and pages in
a single skb, and break invariants. Let's reject devmem tx without
zero-copy.

[...]

Here is the summary with links:
  - [net,v2,1/1] tcp: reject non zerocopy devmem tx
    https://git.kernel.org/netdev/net/c/125755776bc6

You are awesome, thank you!
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