Re: [PATCH] vhost: fix segfault on bad descriptor address.

[Ilya Maximets <i.maximets@samsung.com>]

Hi, Rich.
Thank you for testing and analysing.

On 01.06.2016 01:06, Rich Lane wrote:
> On Fri, May 20, 2016 at 5:50 AM, Ilya Maximets <i.maximets@samsung.com <mailto:i.maximets@samsung.com>> wrote:
> 
>     In current implementation guest application can reinitialize vrings
>     by executing start after stop. In the same time host application
>     can still poll virtqueue while device stopped in guest and it will
>     crash with segmentation fault while vring reinitialization because
>     of dereferencing of bad descriptor addresses.
> 
> 
> I see a performance regression with this patch at large packet sizes (> 768 bytes). rte_vhost_enqueue_burst is consuming 10% more cycles. Strangely, there's actually a ~1% performance improvement at small packet sizes.
> 
> The regression happens with GCC 4.8.4 and 5.3.0, but not 6.1.1.
> 
> AFAICT this is just the compiler generating bad code. One difference is that it's storing the offset on the stack instead of in a register. A workaround is to move the !desc_addr check outside the unlikely macros.
> 
>     --- a/lib/librte_vhost/vhost_rxtx.c
>     +++ b/lib/librte_vhost/vhost_rxtx.c
>     @@ -147,10 +147,10 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
>             struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
> 
>             desc = &vq->desc[desc_idx];
>     -       if (unlikely(desc->len < vq->vhost_hlen))
>     +       desc_addr = gpa_to_vva(dev, desc->addr);
>     +       if (unlikely(desc->len < vq->vhost_hlen || !desc_addr))
> 
> 
>  Workaround: change to "if (unlikely(desc->len < vq->vhost_hlen) || !desc_addr)".
> 
>                     return -1;
> 
> 
>     -       desc_addr = gpa_to_vva(dev, desc->addr);
>             rte_prefetch0((void *)(uintptr_t)desc_addr);
> 
>             virtio_enqueue_offload(m, &virtio_hdr.hdr);
>     @@ -184,6 +184,9 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
> 
>                             desc = &vq->desc[desc->next];
>                             desc_addr   = gpa_to_vva(dev, desc->addr);
>     +                       if (unlikely(!desc_addr))
> 
> 
> Workaround: change to "if (!desc_addr)".
>  
> 
>     +                               return -1;
>     +
>                             desc_offset = 0;
>                             desc_avail  = desc->len;
>                     }
> 

What about other places? Is there same issues or it's only inside copy_mbuf_to_desc() ?

Best regards, Ilya Maximets.