Subject: Re: [RFC PATCH v1] virtio_pci: only store successfully populated virtio_pci_vq_info

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 21 Apr 2026 17:51:11 -0400

On Tue, Apr 21, 2026 at 02:47:32PM -0700, Link Lin wrote:
> Hi everyone,
> 
> Friendly ping. Apologies if you are getting this the second time - my
> last ping wasn't in plain text mode and got rejected by some mailing
> lists.
> 
> Please let me know if anyone has had a chance to look at this RFC
> patch, or if any changes are needed.
> 
> Thanks,
> Link
> 
> On Tue, Apr 21, 2026 at 2:24 PM Link Lin <[email-masked]> wrote:
> >
> > Hi everyone,
> >
> > Friendly ping on this RFC patch. Please let me know if anyone has had a chance to look at this, or if any changes are needed.
> >
> > Thanks,
> > Link
> >
> > On Tue, Apr 7, 2026 at 2:25 PM Link Lin <[email-masked]> wrote:
> >>
> >> In environments where free page reporting is disabled, a kernel
> >> panic is triggered when tearing down the virtio_balloon module:
> >>
> >>   [12261.808190] Call trace:
> >>   [12261.808471]  __list_del_entry_valid_or_report+0x18/0xe0
> >>   [12261.809064]  vp_del_vqs+0x12c/0x270
> >>   [12261.809462]  remove_common+0x80/0x98 [virtio_balloon]
> >>   [12261.810034]  virtballoon_remove+0xfc/0x158 [virtio_balloon]
> >>   [12261.810663]  virtio_dev_remove+0x68/0xf8
> >>   [12261.811108]  device_release_driver_internal+0x17c/0x278
> >>   [12261.811701]  driver_detach+0xd4/0x138
> >>   [12261.812117]  bus_remove_driver+0x90/0xd0
> >>   [12261.812562]  driver_unregister+0x40/0x70
> >>   [12261.813006]  unregister_virtio_driver+0x20/0x38
> >>   [12261.813518]  cleanup_module+0x20/0x7a8 [virtio_balloon]
> >>   [12261.814109]  __arm64_sys_delete_module+0x278/0x3d0
> >>   [12261.814654]  invoke_syscall+0x5c/0x120
> >>   [12261.815086]  el0_svc_common+0x90/0xf8
> >>   [12261.815506]  do_el0_svc+0x2c/0x48
> >>   [12261.815883]  el0_svc+0x3c/0xa8
> >>   [12261.816235]  el0t_64_sync_handler+0x8c/0x108
> >>   [12261.816724]  el0t_64_sync+0x198/0x1a0
> >>
> >> The issue originates in vp_find_vqs_intx(). It kzalloc_objs() based
> >> on the nvqs count provided by the caller, virtio_balloon::init_vqs().
> >> However, it is not always the case that all nvqs number of
> >> virtio_pci_vq_info objects will be properly populated.
> >>
> >> For example, when VIRTIO_BALLOON_F_FREE_PAGE_HINT is absent, the
> >> VIRTIO_BALLOON_VQ_FREE_PAGE-th item in the vp_dev->vqs array is
> >> actually never populated, and is still a zeroe-initialized
> >> virtio_pci_vq_info object, which is eventually going to trigger
> >> a __list_del_entry_valid_or_report() crash.
> >>
> >> Tested by applying this patch to a guest VM kernel with the
> >> VIRTIO_BALLOON_F_REPORTING feature enabled and the
> >> VIRTIO_BALLOON_F_FREE_PAGE_HINT feature disabled.
> >> Without this patch, unloading the virtio_balloon module triggers a panic.
> >> With this patch, no panic is observed.
> >>
> >> The fix is to use queue_idx to handle the case that vp_find_vqs_intx()
> >> skips vp_setup_vq() when caller provided null vqs_info[i].name, when
> >> the caller doesn't populate all nvqs number of virtqueue_info objects.
> >> Invariantly queue_idx is the correct index to store a successfully
> >> created and populated virtio_pci_vq_info object. As a result, now
> >> a virtio_pci_device object only stores queue_idx number of valid
> >> virtio_pci_vq_info objects in its vqs array when the for-loop over
> >> nvqs finishes (of course, without goto out_del_vqs).
> >>
> >> vp_find_vqs_msix() has similar issue, so fix it in the same way.
> >>
> >> This patch is marked as RFC because we are uncertain if any virtio-pci
> >> code implicitly requires virtio_pci_device's vqs array to always
> >> contain nvqs number of virtio_pci_vq_info objects, and to store
> >> zero-initialized virtio_pci_vq_info objects. We have not observed
> >> any issues in our testing, but insights or alternatives are welcome!
> >>
> >> Signed-off-by: Link Lin <[email-masked]>
> >> Co-developed-by: Jiaqi Yan <[email-masked]>
> >> Signed-off-by: Jiaqi Yan <[email-masked]>
> >> ---
> >>  drivers/virtio/virtio_pci_common.c | 10 ++++++----
> >>  1 file changed, 6 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> >> index da97b6a988de..9b32301529e5 100644
> >> --- a/drivers/virtio/virtio_pci_common.c
> >> +++ b/drivers/virtio/virtio_pci_common.c
> >> @@ -423,14 +423,15 @@ static int vp_find_vqs_msix(struct virtio_device *vdev, unsigned int nvqs,
> >>                         vqs[i] = NULL;
> >>                         continue;
> >>                 }
> >> -               vqs[i] = vp_find_one_vq_msix(vdev, queue_idx++, vqi->callback,
> >> +               vqs[i] = vp_find_one_vq_msix(vdev, queue_idx, vqi->callback,
> >>                                              vqi->name, vqi->ctx, false,
> >>                                              &allocated_vectors, vector_policy,
> >> -                                            &vp_dev->vqs[i]);
> >> +                                            &vp_dev->vqs[queue_idx]);
> >>                 if (IS_ERR(vqs[i])) {
> >>                         err = PTR_ERR(vqs[i]);
> >>                         goto error_find;
> >>                 }
> >> +               ++queue_idx;
> >>         }
> >>
> >>         if (!avq_num)
> >> @@ -485,13 +486,14 @@ static int vp_find_vqs_intx(struct virtio_device *vdev, unsigned int nvqs,
> >>                         vqs[i] = NULL;
> >>                         continue;
> >>                 }
> >> -               vqs[i] = vp_setup_vq(vdev, queue_idx++, vqi->callback,
> >> +               vqs[i] = vp_setup_vq(vdev, queue_idx, vqi->callback,
> >>                                      vqi->name, vqi->ctx,
> >> -                                    VIRTIO_MSI_NO_VECTOR, &vp_dev->vqs[i]);
> >> +                                    VIRTIO_MSI_NO_VECTOR, &vp_dev->vqs[queue_idx]);
> >>                 if (IS_ERR(vqs[i])) {
> >>                         err = PTR_ERR(vqs[i]);
> >>                         goto out_del_vqs;
> >>                 }
> >> +               ++queue_idx;
> >>         }
> >>
> >>         if (!avq_num)
> >> --
> >> 2.53.0.1213.gd9a14994de-goog

I have this in my tree:

https://lore.kernel.org/all/[email-masked]/

same?