Re: [PATCH net-next V3] net: restore the iterator to its original state when an error occurs

[Jakub Kicinski <kuba@kernel.org>]

CC: Will, as this looks like ZC side of 7fb1291257ea1e27

On Thu,  4 Dec 2025 00:03:39 +0800 Edward Adam Davis wrote:
> In zerocopy_fill_skb_from_iter(), if two copy operations are performed
> and the first one succeeds while the second one fails, it returns a
> failure but the count in iterator has already been decremented due to
> the first successful copy. This ultimately affects the local variable
> rest_len in virtio_transport_send_pkt_info(), causing the remaining
> count in rest_len to be greater than the actual iterator count. As a
> result, packet sending operations continue even when the iterator count
> is zero, which further leads to skb->len being 0 and triggers the warning
> reported by syzbot [1].
> 
> Therefore, if the zerocopy operation fails, we should revert the iterator
> to its original state.
> 
> The iov_iter_revert() in skb_zerocopy_iter_stream() is no longer needed
> and has been removed.
> 
> [1]
> 'send_pkt()' returns 0, but 4096 expected
> WARNING: net/vmw_vsock/virtio_transport_common.c:430 at virtio_transport_send_pkt_info+0xd1e/0xef0 net/vmw_vsock/virtio_transport_common.c:428, CPU#1: syz.0.17/5986
> Call Trace:
>  virtio_transport_stream_enqueue net/vmw_vsock/virtio_transport_common.c:1113 [inline]
>  virtio_transport_seqpacket_enqueue+0x143/0x1c0 net/vmw_vsock/virtio_transport_common.c:841
>  vsock_connectible_sendmsg+0xabf/0x1040 net/vmw_vsock/af_vsock.c:2158
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg+0x21c/0x270 net/socket.c:746
> 
> Reported-by: syzbot+28e5f3d207b14bae122a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=28e5f3d207b14bae122a
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
> v3:
>   - fix test tcp_zerocopy_maxfrags timeout
> v2: https://lore.kernel.org/all/tencent_BA768766163C533724966E36344AAE754709@qq.com/
>   - Remove iov_iter_revert() in skb_zerocopy_iter_stream()
> v1: https://lore.kernel.org/all/tencent_387517772566B03DBD365896C036264AA809@qq.com/

Have you investigated the other callers? Given problems with previous
version of this patch I'm worried you have not. If you did please extend
the commit message with the appropriate explanation.

Alternative would be to add a _full() flavor of this API, but not sure
if other callers actually care.

> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index c285c6465923..3a612ebbbe80 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -748,9 +748,13 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
>  			    size_t length,
>  			    struct net_devmem_dmabuf_binding *binding)
>  {
> +	struct iov_iter_state state;

nit: if you respin move this one line down

>  	unsigned long orig_size = skb->truesize;
>  	unsigned long truesize;
> -	int ret;
> +	int ret, orig_len;
> +
> +	iov_iter_save_state(from, &state);
> +	orig_len = skb->len;
>  
>  	if (msg && msg->msg_ubuf && msg->sg_from_iter)
>  		ret = msg->sg_from_iter(skb, from, length);
> @@ -759,6 +763,9 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
>  	else
>  		ret = zerocopy_fill_skb_from_iter(skb, from, length);
>  
> +	if (ret == -EFAULT || (ret == -EMSGSIZE && skb->len == orig_len))

I'd think that for the purpose of reverting iter the second part of
this condition is completely moot. If skb len didn't change there should
be nothing to revert?

> +		iov_iter_restore(from, &state);
> +
>  	truesize = skb->truesize - orig_size;
>  	if (sk && sk->sk_type == SOCK_STREAM) {
>  		sk_wmem_queued_add(sk, truesize);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index a00808f7be6a..7b8836f668b7 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1908,7 +1908,6 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
>  		struct sock *save_sk = skb->sk;
>  
>  		/* Streams do not free skb on error. Reset to prev state. */
> -		iov_iter_revert(&msg->msg_iter, skb->len - orig_len);
>  		skb->sk = sk;
>  		___pskb_trim(skb, orig_len);
>  		skb->sk = save_sk;
-- 
pw-bot: cr