linux-kernel - Re: [PATCH 1/3] virtio_ring: always warn when descriptor chain exceeds queue size

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Message-ID: <20210322041414-mutt-send-email-mst@kernel.org>
Date:   Mon, 22 Mar 2021 04:17:33 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Connor Kuehl <ckuehl@...hat.com>, virtio-fs@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, stefanha@...hat.com,
        vgoyal@...hat.com, miklos@...redi.hu
Subject: Re: [PATCH 1/3] virtio_ring: always warn when descriptor chain
 exceeds queue size

On Mon, Mar 22, 2021 at 11:22:15AM +0800, Jason Wang wrote:
> 
> 在 2021/3/18 下午9:52, Connor Kuehl 写道:
> >  From section 2.6.5.3.1 (Driver Requirements: Indirect Descriptors)
> > of the virtio spec:
> > 
> >    "A driver MUST NOT create a descriptor chain longer than the Queue
> >    Size of the device."
> > 
> > This text suggests that the warning should trigger even if
> > indirect descriptors are in use.
> 
> 
> So I think at least the commit log needs some tweak.
> 
> For split virtqueue. We had:
> 
> 2.6.5.2 Driver Requirements: The Virtqueue Descriptor Table
> 
> Drivers MUST NOT add a descriptor chain longer than 2^32 bytes in total;
> this implies that loops in the descriptor chain are forbidden!
> 
> 2.6.5.3.1 Driver Requirements: Indirect Descriptors
> 
> A driver MUST NOT create a descriptor chain longer than the Queue Size of
> the device.
> 
> If I understand the spec correctly, the check is only needed for a single
> indirect descriptor table?
> 
> For packed virtqueue. We had:
> 
> 2.7.17 Driver Requirements: Scatter-Gather Support
> 
> A driver MUST NOT create a descriptor list longer than allowed by the
> device.
> 
> A driver MUST NOT create a descriptor list longer than the Queue Size.
> 
> 2.7.19 Driver Requirements: Indirect Descriptors
> 
> A driver MUST NOT create a descriptor chain longer than allowed by the
> device.
> 
> So it looks to me the packed part is fine.
> 
> Note that if I understand the spec correctly 2.7.17 implies 2.7.19.
> 
> Thanks

It would be quite strange for packed and split to differ here:
so for packed would you say there's no limit on # of descriptors at all?

I am guessing I just forgot to move this part from
the format specific to the common part of the spec.

This needs discussion in the TC mailing list - want to start a thread
there?



> 
> > 
> > Reported-by: Stefan Hajnoczi <stefanha@...hat.com>
> > Signed-off-by: Connor Kuehl <ckuehl@...hat.com>
> > ---
> >   drivers/virtio/virtio_ring.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 71e16b53e9c1..1bc290f9ba13 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -444,11 +444,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >   	head = vq->free_head;
> > +	WARN_ON_ONCE(total_sg > vq->split.vring.num);
> > +
> >   	if (virtqueue_use_indirect(_vq, total_sg))
> >   		desc = alloc_indirect_split(_vq, total_sg, gfp);
> >   	else {
> >   		desc = NULL;
> > -		WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);
> >   	}
> >   	if (desc) {
> > @@ -1118,6 +1119,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >   	BUG_ON(total_sg == 0);
> > +	WARN_ON_ONCE(total_sg > vq->packed.vring.num);
> > +
> >   	if (virtqueue_use_indirect(_vq, total_sg))
> >   		return virtqueue_add_indirect_packed(vq, sgs, total_sg,
> >   				out_sgs, in_sgs, data, gfp);
> > @@ -1125,8 +1128,6 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >   	head = vq->packed.next_avail_idx;
> >   	avail_used_flags = vq->packed.avail_used_flags;
> > -	WARN_ON_ONCE(total_sg > vq->packed.vring.num && !vq->indirect);
> > -
> >   	desc = vq->packed.vring.desc;
> >   	i = head;
> >   	descs_used = total_sg;