[PATCH 1/1] vhost: fix a double fetch when dequeue offloading

[Wangyunjian(wangyunjian,TongTu)]

> -----Original Message-----
> From: David Marchand [mailto:david.marchand at redhat.com]
> Sent: Thursday, December 19, 2024 4:24 PM
> To: Wangyunjian(wangyunjian,TongTu) <wangyunjian at huawei.com>;
> maxime.coquelin at redhat.com
> Cc: dev at dpdk.org; chenbox at nvidia.com; Lilijun (Jerry)
> <jerry.lilijun at huawei.com>; xiawei (H) <xiawei40 at huawei.com>;
> wangzengyuan <wangzengyuan at huawei.com>; stable at dpdk.org
> Subject: Re: [PATCH 1/1] vhost: fix a double fetch when dequeue offloading
> 
> On Thu, Dec 19, 2024 at 7:38 AM Yunjian Wang <wangyunjian at huawei.com>
> wrote:
> >
> > The hdr->csum_start does two successive reads from user space to read a
> > variable length data structure. The result overflow if the data structure
> > changes between the two reads.
> >
> > To fix this, we can prevent double fetch issue by copying virtio_hdr to
> > the temporary variable.
> >
> > Fixes: 4dc4e33ffa10 ("net/virtio: fix Rx checksum calculation")
> > Cc: stable at dpdk.org
> >
> > Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
> > ---
> >  lib/vhost/virtio_net.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> > index 69901ab3b5..5c40ae7069 100644
> > --- a/lib/vhost/virtio_net.c
> > +++ b/lib/vhost/virtio_net.c
> > @@ -2914,10 +2914,12 @@ desc_to_mbuf(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> >                          * in a contiguous virtual area.
> >                          */
> >                         copy_vnet_hdr_from_desc(&tmp_hdr,
> buf_vec);
> > -                       hdr = &tmp_hdr;
> >                 } else {
> > -                       hdr = (struct virtio_net_hdr
> *)((uintptr_t)buf_vec[0].buf_addr);
> > +                       rte_memcpy((void *)(uintptr_t)&tmp_hdr,
> > +                               (void
> *)(uintptr_t)buf_vec[0].buf_addr,
> > +                               sizeof(struct virtio_net_hdr));
> >                 }
> > +               hdr = &tmp_hdr;
> >         }
> 
> This will need some benchmark, as I remember putting rte_memcpy in
> inlined helpers had some performance impact.
> 
> Instead, I would call copy_vnet_hdr_from_desc unconditionnally, and
> store in a struct virtio_net_hdr hdr variable (+ a has_vnet_hdr
> boolean to indicate validity).
> Something like:
>         if (virtio_net_with_host_offload(dev)) {
> -               if (unlikely(buf_vec[0].buf_len < sizeof(struct
> virtio_net_hdr))) {
> -                       /*
> -                        * No luck, the virtio-net header doesn't fit
> -                        * in a contiguous virtual area.
> -                        */
> -                       copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
> -                       hdr = &tmp_hdr;
> -               } else {
> -                       hdr = (struct virtio_net_hdr
> *)((uintptr_t)buf_vec[0].buf_addr);
> -               }
> +               copy_vnet_hdr_from_desc(&hdr, buf_vec);
> +               has_vnet_hdr = true;
>         }
> 
> (besides, in copy_vnet_hdr_from_desc, the while (cond) {} loop could
> be changed to do a do {} while (cond), and that approach requires
> performance numbers too)

How about this?
@@ -2904,8 +2904,8 @@ desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
        uint32_t hdr_remain = dev->vhost_hlen;
        uint32_t cpy_len;
        struct rte_mbuf *cur = m, *prev = m;
-       struct virtio_net_hdr tmp_hdr;
-       struct virtio_net_hdr *hdr = NULL;
+       bool has_vnet_hdr = false;
+       struct virtio_net_hdr hdr;
        uint16_t vec_idx;
        struct vhost_async *async = vq->async;
        struct async_inflight_info *pkts_info;
@@ -2921,11 +2921,11 @@ desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
                         * No luck, the virtio-net header doesn't fit
                         * in a contiguous virtual area.
                         */
-                       copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
-                       hdr = &tmp_hdr;
+                       copy_vnet_hdr_from_desc(&hdr, buf_vec);
                } else {
-                       hdr = (struct virtio_net_hdr *)((uintptr_t)buf_vec[0].buf_addr);
+                       hdr = *(struct virtio_net_hdr *)((uintptr_t)buf_vec[0].buf_addr);
                }
+               has_vnet_hdr = true;
        }


> 
> 
> >
> >         for (vec_idx = 0; vec_idx < nr_vec; vec_idx++) {
> > @@ -3363,7 +3365,7 @@ virtio_dev_tx_batch_packed(struct virtio_net
> *dev,
> >  {
> >         uint16_t avail_idx = vq->last_avail_idx;
> >         uint32_t buf_offset = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> > -       struct virtio_net_hdr *hdr;
> > +       struct virtio_net_hdr hdr;
> >         uintptr_t desc_addrs[PACKED_BATCH_SIZE];
> >         uint16_t ids[PACKED_BATCH_SIZE];
> >         uint16_t i;
> > @@ -3382,8 +3384,9 @@ virtio_dev_tx_batch_packed(struct virtio_net
> *dev,
> >
> >         if (virtio_net_with_host_offload(dev)) {
> >                 vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> > -                       hdr = (struct virtio_net_hdr *)(desc_addrs[i]);
> > -                       vhost_dequeue_offload(dev, hdr, pkts[i],
> legacy_ol_flags);
> > +                       rte_memcpy((void *)(uintptr_t)&hdr,
> > +                               (void *)(uintptr_t)desc_addrs[i],
> sizeof(struct virtio_net_hdr));
> > +                       vhost_dequeue_offload(dev, &hdr, pkts[i],
> legacy_ol_flags);
> >                 }
> >         }
> 
> Here too, there may be an impact with adding rte_memcpy.
> Just do a copy like:
> 
>         if (virtio_net_with_host_offload(dev)) {
> +               struct virtio_net_hdr hdr;
> +
>                 vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> -                       hdr = (struct virtio_net_hdr *)(desc_addrs[i]);
> -                       vhost_dequeue_offload(dev, hdr, pkts[i],
> legacy_ol_flags);
> +                       hdr = *(struct virtio_net_hdr *)(desc_addrs[i]);
> +                       vhost_dequeue_offload(dev, &hdr, pkts[i],
> legacy_ol_flags);
>                 }
> 

Thanks for your suggestion, will include them in next version.

> 
> --
> David Marchand
>