Subject: Re: [PATCH net-next 03/13] virtio_ring: packed: harden dma unmap for indirect

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 12 Sep 2024 03:38:12 -0400

On Thu, Sep 12, 2024 at 02:55:38PM +0800, Xuan Zhuo wrote:
> On Wed, 11 Sep 2024 07:28:36 -0400, "Michael S. Tsirkin" <[email-masked]> wrote:
> > As gcc luckily noted:
> >
> > On Tue, Aug 20, 2024 at 03:33:20PM +0800, Xuan Zhuo wrote:
> > > @@ -1617,23 +1617,24 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > >  	}
> > >
> > >  	if (vq->indirect) {
> > > +		struct vring_desc_extra *extra;
> > >  		u32 len;
> > >
> > >  		/* Free the indirect table, if any, now that it's unmapped. */
> > > -		desc = state->indir_desc;
> > > -		if (!desc)
> >
> > desc is no longer initialized here
> 
> 
> Will fix.
> 
> 
> >
> > > +		extra = state->indir;
> > > +		if (!extra)
> > >  			return;
> > >
> > >  		if (vring_need_unmap_buffer(vq)) {
> > >  			len = vq->packed.desc_extra[id].len;
> > >  			for (i = 0; i < len / sizeof(struct vring_packed_desc);
> > >  					i++)
> > > -				vring_unmap_desc_packed(vq, &desc[i]);
> > > +				vring_unmap_extra_packed(vq, &extra[i]);
> > >  		}
> > >  		kfree(desc);
> >
> >
> > but freed here
> >
> > > -		state->indir_desc = NULL;
> > > +		state->indir = NULL;
> > >  	} else if (ctx) {
> > > -		*ctx = state->indir_desc;
> > > +		*ctx = state->indir;
> > >  	}
> > >  }
> >
> >
> > It seems unlikely this was always 0 on all paths with even
> > a small amount of stress, so now I question how this was tested.
> > Besides, do not ignore compiler warnings, and do not tweak code
> > to just make compiler shut up - they are your friend.
> 
> I agree.
> 
> Normally I do this by make W=12, but we have too many message,
> so I missed this.
> 
> 	make W=12 drivers/net/virtio_net.o drivers/virtio/virtio_ring.o
> 
> If not W=12, then I did not get any warning message.
> How do you get the message quickly?
> 
> Thanks.

If you stress test this for a long enough time, and with
debug enabled, you will see a crash.

> > > --
> > > 2.32.0.3.g01195cf9f