Subject: Re: [PATCH net] vsock/virtio: fix skb overhead overflow on 32-bit builds

From: David Laight <[email-masked]>
Date: Thu, 21 May 2026 18:13:48 +0100

On Thu, 21 May 2026 14:47:32 +0200
Stefano Garzarella <[email-masked]> wrote:

> From: Stefano Garzarella <[email-masked]>
> 
> On 32-bit architectures, both skb_queue_len() and SKB_TRUESIZE(0) evaluate
> to 32-bit values. The multiplication can overflow before being assigned to
> the u64 skb_overhead variable, making the skb overhead check ineffective.
> 
> Cast skb_queue_len() to u64 so the multiplication is always performed in
> 64-bit arithmetic.
> 
> This issue was reported by Sashiko while reviewing another patch.
> 
> Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
> Closes: https://sashiko.dev/#/patchset/20260518090656.134588-1-sgarzare%40redhat.com
> Cc: [email-masked]
> Signed-off-by: Stefano Garzarella <[email-masked]>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index df3b418e0392..71198bf23fc4 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -417,7 +417,7 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
>  					u32 len)
>  {
> -	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
> +	u64 skb_overhead = ((u64)skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);

I was thinking this should use mul_u32_u32().
But that is all moot.
'skb_overhead' is a memory size in bytes, 'unsigned long' it more than big enough.
No need for 64bit maths on 32bit.

-- David

>  
>  	/* Allow at most buf_alloc * 2 total budget (payload + overhead),
>  	 * similar to how SO_RCVBUF is doubled to reserve space for sk_buff