On Mon, Aug 15, 2022 at 01:53:30PM -0700, Andres Freund wrote:> Hi, > > On 2022-08-15 16:21:51 -0400, Michael S. Tsirkin wrote: > > On Mon, Aug 15, 2022 at 10:46:17AM -0700, Andres Freund wrote: > > > Hi, > > > > > > On 2022-08-15 12:50:52 -0400, Michael S. Tsirkin wrote: > > > > On Mon, Aug 15, 2022 at 09:45:03AM -0700, Andres Freund wrote: > > > > > Hi, > > > > > > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote: > > > > > > OK so this gives us a quick revert as a solution for now. > > > > > > Next, I would appreciate it if you just try this simple hack. > > > > > > If it crashes we either have a long standing problem in virtio > > > > > > code or more likely a gcp bug where it can't handle smaller > > > > > > rings than what device requestes. > > > > > > Thanks! > > > > > > > > > > I applied the below and the problem persists. > > > > > > > > > > [...] > > > > > > > > Okay! > > > > > > Just checking - I applied and tested this atop 6.0-rc1, correct? Or did you > > > want me to test it with the 762faee5a267 reverted? I guess what you're trying > > > to test if a smaller queue than what's requested you'd want to do so without > > > the problematic patch applied... > > > > > > > > > Either way, I did this, and there are no issues that I could observe. No > > > oopses, no broken networking. But: > > > > > > To make sure it does something I added a debugging printk - which doesn't show > > > up. I assume this is at a point at least earlyprintk should work (which I see > > > getting enabled via serial)? > > > > > > Sorry if I was unclear. I wanted to know whether the change somehow > > exposes a driver bug or a GCP bug. So what I wanted to do is to test > > this patch on top of *5.19*, not on top of the revert. > > Right, the 5.19 part was clear, just the earlier test: > > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote: > > > > > > OK so this gives us a quick revert as a solution for now. > > > > > > Next, I would appreciate it if you just try this simple hack. > > > > > > If it crashes we either have a long standing problem in virtio > > > > > > code or more likely a gcp bug where it can't handle smaller > > > > > > Thanks! > > I wasn't sure about. > > After I didn't see any effect on 5.19 + your patch, I grew a bit suspicious > and added the printks. > > > > Yes I think printk should work here. > > The reason the debug patch didn't change anything, and that my debug printk > didn't show, is that gcp uses the legacy paths...Wait a second. Eureka I think! So I think GCP is not broken. I think what's broken is this patch: commit cdb44806fca2d0ad29ca644cbf1505433902ee0c Author: Xuan Zhuo <xuanzhuo at linux.alibaba.com> Date: Mon Aug 1 14:38:54 2022 +0800 virtio_pci: support the arg sizes of find_vqs() Specifically: diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c index 2257f1b3d8ae..d75e5c4e637f 100644 --- a/drivers/virtio/virtio_pci_legacy.c +++ b/drivers/virtio/virtio_pci_legacy.c @@ -112,6 +112,7 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev, unsigned int index, void (*callback)(struct virtqueue *vq), const char *name, + u32 size, bool ctx, u16 msix_vec) { @@ -125,10 +126,13 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev, if (!num || vp_legacy_get_queue_enable(&vp_dev->ldev, index)) return ERR_PTR(-ENOENT); + if (!size || size > num) + size = num; + info->msix_vector = msix_vec; /* create the vring */ - vq = vring_create_virtqueue(index, num, + vq = vring_create_virtqueue(index, size, VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev, true, false, ctx, vp_notify, callback, name); So if you pass the size parameter for a legacy device it will try to make the ring smaller and that is not legal with legacy at all. But the driver treats legacy and modern the same, it allocates a smaller queue anyway. Lo and behold, I pass disable-modern=on to qemu and it happily corrupts memory exactly the same as GCP does. So the new find_vqs API is actually completely broken, it can not work for legacy at all and for added fun there's no way to find out that it's legacy. Maybe we should interpret the patch So I think I will also revert 04ca0b0b16f11faf74fa92468dab51b8372586cd..fe3dc04e31aa51f91dc7f741a5f76cc4817eb5b4> If there were a bug in the legacy path, it'd explain why the problem only > shows on gcp, and not in other situations. > > I'll queue testing the legacy path with the equivalent change. > > - Andres > > > Greetings, > > Andres Freund
On Mon, 15 Aug 2022 17:32:06 -0400, "Michael S. Tsirkin" <mst at redhat.com> wrote:> On Mon, Aug 15, 2022 at 01:53:30PM -0700, Andres Freund wrote: > > Hi, > > > > On 2022-08-15 16:21:51 -0400, Michael S. Tsirkin wrote: > > > On Mon, Aug 15, 2022 at 10:46:17AM -0700, Andres Freund wrote: > > > > Hi, > > > > > > > > On 2022-08-15 12:50:52 -0400, Michael S. Tsirkin wrote: > > > > > On Mon, Aug 15, 2022 at 09:45:03AM -0700, Andres Freund wrote: > > > > > > Hi, > > > > > > > > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote: > > > > > > > OK so this gives us a quick revert as a solution for now. > > > > > > > Next, I would appreciate it if you just try this simple hack. > > > > > > > If it crashes we either have a long standing problem in virtio > > > > > > > code or more likely a gcp bug where it can't handle smaller > > > > > > > rings than what device requestes. > > > > > > > Thanks! > > > > > > > > > > > > I applied the below and the problem persists. > > > > > > > > > > > > [...] > > > > > > > > > > Okay! > > > > > > > > Just checking - I applied and tested this atop 6.0-rc1, correct? Or did you > > > > want me to test it with the 762faee5a267 reverted? I guess what you're trying > > > > to test if a smaller queue than what's requested you'd want to do so without > > > > the problematic patch applied... > > > > > > > > > > > > Either way, I did this, and there are no issues that I could observe. No > > > > oopses, no broken networking. But: > > > > > > > > To make sure it does something I added a debugging printk - which doesn't show > > > > up. I assume this is at a point at least earlyprintk should work (which I see > > > > getting enabled via serial)? > > > > > > > > > Sorry if I was unclear. I wanted to know whether the change somehow > > > exposes a driver bug or a GCP bug. So what I wanted to do is to test > > > this patch on top of *5.19*, not on top of the revert. > > > > Right, the 5.19 part was clear, just the earlier test: > > > > > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote: > > > > > > > OK so this gives us a quick revert as a solution for now. > > > > > > > Next, I would appreciate it if you just try this simple hack. > > > > > > > If it crashes we either have a long standing problem in virtio > > > > > > > code or more likely a gcp bug where it can't handle smaller > > > > > > > Thanks! > > > > I wasn't sure about. > > > > After I didn't see any effect on 5.19 + your patch, I grew a bit suspicious > > and added the printks. > > > > > > > Yes I think printk should work here. > > > > The reason the debug patch didn't change anything, and that my debug printk > > didn't show, is that gcp uses the legacy paths... > > Wait a second. Eureka I think! > > So I think GCP is not broken. > I think what's broken is this patch: > > commit cdb44806fca2d0ad29ca644cbf1505433902ee0c > Author: Xuan Zhuo <xuanzhuo at linux.alibaba.com> > Date: Mon Aug 1 14:38:54 2022 +0800 > > virtio_pci: support the arg sizes of find_vqs() > > > Specifically: > > diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c > index 2257f1b3d8ae..d75e5c4e637f 100644 > --- a/drivers/virtio/virtio_pci_legacy.c > +++ b/drivers/virtio/virtio_pci_legacy.c > @@ -112,6 +112,7 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev, > unsigned int index, > void (*callback)(struct virtqueue *vq), > const char *name, > + u32 size, > bool ctx, > u16 msix_vec) > { > @@ -125,10 +126,13 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev, > if (!num || vp_legacy_get_queue_enable(&vp_dev->ldev, index)) > return ERR_PTR(-ENOENT); > > + if (!size || size > num) > + size = num; > + > info->msix_vector = msix_vec; > > /* create the vring */ > - vq = vring_create_virtqueue(index, num, > + vq = vring_create_virtqueue(index, size, > VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev, > true, false, ctx, > vp_notify, callback, name); > > > > So if you pass the size parameter for a legacy device it will > try to make the ring smaller and that is not legal with > legacy at all. But the driver treats legacy and modern > the same, it allocates a smaller queue anyway. > > > Lo and behold, I pass disable-modern=on to qemu and it happily > corrupts memory exactly the same as GCP does.Yes, I think you are right. Thank you very much.> > > So the new find_vqs API is actually completely broken, it can not work for > legacy at all and for added fun there's no way to find out > that it's legacy. Maybe we should interpret the patch > > So I think I will also revert > > 04ca0b0b16f11faf74fa92468dab51b8372586cd..fe3dc04e31aa51f91dc7f741a5f76cc4817eb5b4 > > > > > > > > > If there were a bug in the legacy path, it'd explain why the problem only > > shows on gcp, and not in other situations. > > > > I'll queue testing the legacy path with the equivalent change. > > > > - Andres > > > > > > Greetings, > > > > Andres Freund >
On Mon, 15 Aug 2022 17:32:06 -0400, Michael wrote:> So if you pass the size parameter for a legacy device it will > try to make the ring smaller and that is not legal with > legacy at all. But the driver treats legacy and modern > the same, it allocates a smaller queue anyway. > > Lo and behold, I pass disable-modern=on to qemu and it happily > corrupts memory exactly the same as GCP does.Ouch! I understand that the host does the actual corruption, but could you think of any additional debug checking in the guest that would caught this in future? Potentially only when KASAN is enabled which can verify validity of memory ranges. Some kind of additional layer of sanity checking. This caused a bit of a havoc for syzbot with almost 100 unique crash signatures, so would be useful to catch such issues more reliably in future. Thanks