On Fri, 18 Oct 2024 15:41:41 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> On Mon, Oct 14, 2024 at 11:12 AM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
> >
> > When the frag just got a page, then may lead to regression on VM.
> > Specially if the sysctl net.core.high_order_alloc_disable value is 1,
> > then the frag always get a page when do refill.
> >
> > 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.
> >
> > Here, when the frag size is not enough, we reduce the buffer len to fix
> > this problem.
> >
> > Fixes: f9dac92ba908 ("virtio_ring: enable premapped mode whatever use_dma_api")
> > Reported-by: "Si-Wei Liu" <si-wei.liu@xxxxxxxxxx>
> > Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
>
> Though this may fix the problem and we need it now, I would like to
> have some tweaks in the future. Basically because of the security
> implication for sharing driver metadata with the device (most IOMMU
> can only do protection at the granule at the page). So we may end up
> with device-triggerable behaviour. For safety, we should use driver
> private memory for DMA metadata.
>
> > ---
> > 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 f8131f92a392..59a99bbaf852 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -926,9 +926,6 @@ 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);
> >
> > if (rq->do_dma) {
> > @@ -2423,6 +2420,9 @@ 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;
> > @@ -2525,6 +2525,12 @@ 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;
>
> This makes me think of another question, how could we guarantee len +
> room is less or equal to PAGE_SIZE. Especially considering we need
> extra head and tailroom for XDP? If we can't it is a bug:
get_mergeable_buf_len() do this.
>
> """
> /**
> * skb_page_frag_refill - check that a page_frag contains enough room
> * @sz: minimum size of the fragment we want to get
> * @pfrag: pointer to page_frag
> * @gfp: priority for memory allocation
> *
> * Note: While this allocator tries to use high order pages, there is
> * no guarantee that allocations succeed. Therefore, @sz MUST be
> * less or equal than PAGE_SIZE.
> */
> """
>
> > +
> > + if (!alloc_frag->offset && len + room + sizeof(struct virtnet_rq_dma) > alloc_frag->size)
> > + len -= sizeof(struct virtnet_rq_dma);
>
> Any reason we need to check alloc_frag->offset?
We just need to check when the page of the alloc frag is new.
In this case, we need to allocate space to store dma meta.
If the offset > 0, then we do not need to allocate extra space,
so it is safe.
>
> > +
> > buf = virtnet_rq_alloc(rq, len + room, gfp);
>
> Btw, as pointed out in previous review, we should have a consistent API:
>
> 1) hide the alloc_frag like virtnet_rq_alloc()
>
> or
>
> 2) pass the alloc_frag to virtnet_rq_alloc()
Now we need to check len+room before calling skb_page_frag_refill()
so we must move virtnet_rq_alloc() outside virtnet_rq_alloc().
Thanks
>
> > if (unlikely(!buf))
> > return -ENOMEM;
> > --
> > 2.32.0.3.g01195cf9f
> >
>
> Thanks
>
