virtio_pci: fix vq info pointer lookup via wrong index

Unbinding a virtio balloon device:

    echo virtio0 > /sys/bus/virtio/drivers/virtio_balloon/unbind

triggers a NULL pointer dereference. The dmesg says:

    BUG: kernel NULL pointer dereference, address: 0000000000000008
    [...]
    RIP: 0010:__list_del_entry_valid_or_report+0x5/0xf0
    Call Trace:
    <TASK>
    vp_del_vqs+0x121/0x230
    remove_common+0x135/0x150
    virtballoon_remove+0xee/0x100
    virtio_dev_remove+0x3b/0x80
    device_release_driver_internal+0x187/0x2c0
    unbind_store+0xb9/0xe0
    kernfs_fop_write_iter.llvm.11660790530567441834+0xf6/0x180
    vfs_write+0x2a9/0x3b0
    ksys_write+0x5c/0xd0
    do_syscall_64+0x54/0x230
    entry_SYSCALL_64_after_hwframe+0x29/0x31
    [...]
    </TASK>

The virtio_balloon device registers 5 queues (inflate, deflate, stats,
free_page, reporting) but only the first two are unconditional. The
stats, free_page and reporting queues are each conditional on their
respective feature bits. When any of these features are absent, the
corresponding vqs_info entry has name == NULL, creating holes in the
array.

The root cause is an indexing mismatch introduced when vq info storage
was changed to be passed as an argument. vp_find_vqs_msix() and
vp_find_vqs_intx() store the info pointer at vp_dev->vqs[i], where 'i'
is the caller's sparse array index. However, the virtqueue itself gets
vq->index assigned from queue_idx, a dense index that skips NULL
entries. When holes exist, 'i' and queue_idx diverge. Later,
vp_del_vqs() looks up info via vp_dev->vqs[vq->index] using the dense
index into the sparsely-populated array, and hits NULL.

Fix this by storing info at vp_dev->vqs[queue_idx] instead of
vp_dev->vqs[i], so the store index matches the lookup index
(vq->index). Apply the fix to both the MSIX and INTX paths.

Cc: Yichun Zhang <yichun@openresty.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Cc: stable@vger.kernel.org # v6.11+
Tested-by: Yuka <yuka@umeyashiki.org>
Fixes: 89a1c435aec2 ("virtio_pci: pass vq info as an argument to vp_setup_vq()")
Signed-off-by: Ammar Faizi <ammarfaizi2@openresty.com>
Message-Id: <20260315141808.547081-1-ammarfaizi2@openresty.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_pci_common.c

diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
index da97b6a..164f480 100644
--- a/drivers/virtio/virtio_pci_common.c
+++ b/drivers/virtio/virtio_pci_common.c
@@ -423,10 +423,11 @@ 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]);
+		queue_idx++;
 		if (IS_ERR(vqs[i])) {
 			err = PTR_ERR(vqs[i]);
 			goto error_find;
@@ -485,9 +486,10 @@ 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]);
+		queue_idx++;
 		if (IS_ERR(vqs[i])) {
 			err = PTR_ERR(vqs[i]);
 			goto out_del_vqs;