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

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/19/26 18:45, Kaifeng Wang wrote:
> 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)) {

I wonder why it does sockc_err checks like that for devmem, e.g. instead of
failing earlier. Maybe Mina remembers?

> +		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;

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

-- 
Pavel Begunkov