Subject: Re: [PATCH net] virtio-net: fix overflow inside virtnet_rq_alloc

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 6 Sep 2024 04:43:29 -0400

On Tue, Aug 20, 2024 at 03:19:13PM +0800, Xuan Zhuo wrote:
> leads to regression on VM with the sysctl value of:
> 
> - net.core.high_order_alloc_disable=1
> 
> which could see reliable crashes or scp failure (scp a file 100M in size
> to VM):
> 
> The issue is that the virtnet_rq_dma takes up 16 bytes at the beginning
> of a new frag. When the frag size is larger than PAGE_SIZE,
> everything is fine. However, if the frag is only one page and the
> total size of the buffer and virtnet_rq_dma is larger than one page, an
> overflow may occur. In this case, if an overflow is possible, I adjust
> the buffer size. If net.core.high_order_alloc_disable=1, the maximum
> buffer size is 4096 - 16. If net.core.high_order_alloc_disable=0, only
> the first buffer of the frag is affected.
> 
> Fixes: f9dac92ba908 ("virtio_ring: enable premapped mode whatever use_dma_api")
> Reported-by: "Si-Wei Liu" <[email-masked]>
> Closes: http://lore.kernel.org/all/[email-masked]
> Signed-off-by: Xuan Zhuo <[email-masked]>

Guys where are we going with this? We have a crasher right now,
if this is not fixed ASAP I'd have to revert a ton of
work Xuan Zhuo just did.

> ---
>  drivers/net/virtio_net.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c6af18948092..e5286a6da863 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -918,9 +918,6 @@ static void *virtnet_rq_alloc(struct receive_queue *rq, u32 size, gfp_t gfp)
>  	void *buf, *head;
>  	dma_addr_t addr;
>  
> -	if (unlikely(!skb_page_frag_refill(size, alloc_frag, gfp)))
> -		return NULL;
> -
>  	head = page_address(alloc_frag->page);
>  
>  	dma = head;
> @@ -2421,6 +2418,9 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  	len = SKB_DATA_ALIGN(len) +
>  	      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>  
> +	if (unlikely(!skb_page_frag_refill(len, &rq->alloc_frag, gfp)))
> +		return -ENOMEM;
> +
>  	buf = virtnet_rq_alloc(rq, len, gfp);
>  	if (unlikely(!buf))
>  		return -ENOMEM;
> @@ -2521,6 +2521,12 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>  	 */
>  	len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);
>  
> +	if (unlikely(!skb_page_frag_refill(len + room, alloc_frag, gfp)))
> +		return -ENOMEM;
> +
> +	if (!alloc_frag->offset && len + room + sizeof(struct virtnet_rq_dma) > alloc_frag->size)
> +		len -= sizeof(struct virtnet_rq_dma);
> +
>  	buf = virtnet_rq_alloc(rq, len + room, gfp);
>  	if (unlikely(!buf))
>  		return -ENOMEM;
> -- 
> 2.32.0.3.g01195cf9f