Make vhost-user-blk backwards compatible when migrating from older VMs
running with modern features turned off, the same way it was done for
virtio-blk in 20764be0421c ("virtio-blk: set config size depending on the features enabled")

It's currently impossible to migrate from an older VM with
vhost-user-blk (with disable-legacy=off) because of errors like this:

qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10 read: 41 device: 1 cmask: ff wmask: 80 w1cmask:0
qemu-system-x86_64: Failed to load PCIDevice:config
qemu-system-x86_64: Failed to load virtio-blk:virtio
qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:05.0:00.0:02.0/virtio-blk'
qemu-system-x86_64: load of migration failed: Invalid argument

This is caused by the newer (destination) VM requiring a bigger BAR0
alignment because it has to cover a bigger configuration space, which
isn't actually needed since those additional config fields are not
active (write-zeroes/discard).

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
---
 hw/block/vhost-user-blk.c          | 15 ++++++++-------
 include/hw/virtio/vhost-user-blk.h |  1 +
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 64f3457373..d18a7a2cd4 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -23,6 +23,7 @@
 #include "hw/qdev-core.h"
 #include "hw/qdev-properties.h"
 #include "hw/qdev-properties-system.h"
+#include "hw/virtio/virtio-blk-common.h"
 #include "hw/virtio/vhost.h"
 #include "hw/virtio/vhost-user-blk.h"
 #include "hw/virtio/virtio.h"
@@ -63,7 +64,7 @@ static void vhost_user_blk_update_config(VirtIODevice *vdev, uint8_t *config)
     /* Our num_queues overrides the device backend */
     virtio_stw_p(vdev, &s->blkcfg.num_queues, s->num_queues);
 
-    memcpy(config, &s->blkcfg, sizeof(struct virtio_blk_config));
+    memcpy(config, &s->blkcfg, s->config_size);
 }
 
 static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
@@ -96,8 +97,7 @@ static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
     Error *local_err = NULL;
 
     ret = vhost_dev_get_config(dev, (uint8_t *)&blkcfg,
-                               sizeof(struct virtio_blk_config),
-                               &local_err);
+                               s->config_size, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         return ret;
@@ -106,7 +106,7 @@ static int vhost_user_blk_handle_config_change(struct vhost_dev *dev)
     /* valid for resize only */
     if (blkcfg.capacity != s->blkcfg.capacity) {
         s->blkcfg.capacity = blkcfg.capacity;
-        memcpy(dev->vdev->config, &s->blkcfg, sizeof(struct virtio_blk_config));
+        memcpy(dev->vdev->config, &s->blkcfg, s->config_size);
         virtio_notify_config(dev->vdev);
     }
 
@@ -444,7 +444,7 @@ static int vhost_user_blk_realize_connect(VHostUserBlk *s, Error **errp)
     assert(s->connected);
 
     ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
-                               sizeof(struct virtio_blk_config), errp);
+                               s->config_size, errp);
     if (ret < 0) {
         qemu_chr_fe_disconnect(&s->chardev);
         vhost_dev_cleanup(&s->dev);
@@ -489,8 +489,9 @@ static void vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
-    virtio_init(vdev, VIRTIO_ID_BLOCK,
-                sizeof(struct virtio_blk_config));
+    s->config_size = virtio_blk_common_get_config_size(s->host_features);
+
+    virtio_init(vdev, VIRTIO_ID_BLOCK, s->config_size);
 
     s->virtqs = g_new(VirtQueue *, s->num_queues);
     for (i = 0; i < s->num_queues; i++) {
diff --git a/include/hw/virtio/vhost-user-blk.h b/include/hw/virtio/vhost-user-blk.h
index 6252095c45..b7810360b9 100644
--- a/include/hw/virtio/vhost-user-blk.h
+++ b/include/hw/virtio/vhost-user-blk.h
@@ -52,6 +52,7 @@ struct VHostUserBlk {
     bool started_vu;
 
     uint64_t host_features;
+    size_t config_size;
 };
 
 #endif
-- 
2.25.1

On Wed, Aug 24, 2022 at 12:18:37PM +0300, Daniil Tatianin wrote:
> Make vhost-user-blk backwards compatible when migrating from older VMs
> running with modern features turned off, the same way it was done for
> virtio-blk in 20764be0421c ("virtio-blk: set config size depending on the features enabled")
> 
> It's currently impossible to migrate from an older VM with
> vhost-user-blk (with disable-legacy=off) because of errors like this:
> 
> qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10 read: 41 device: 1 cmask: ff wmask: 80 w1cmask:0
> qemu-system-x86_64: Failed to load PCIDevice:config
> qemu-system-x86_64: Failed to load virtio-blk:virtio
> qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:05.0:00.0:02.0/virtio-blk'
> qemu-system-x86_64: load of migration failed: Invalid argument
> 
> This is caused by the newer (destination) VM requiring a bigger BAR0
> alignment because it has to cover a bigger configuration space, which
> isn't actually needed since those additional config fields are not
> active (write-zeroes/discard).
> 
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c          | 15 ++++++++-------
>  include/hw/virtio/vhost-user-blk.h |  1 +
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 64f3457373..d18a7a2cd4 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -23,6 +23,7 @@
>  #include "hw/qdev-core.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/qdev-properties-system.h"
> +#include "hw/virtio/virtio-blk-common.h"
>  #include "hw/virtio/vhost.h"
>  #include "hw/virtio/vhost-user-blk.h"
>  #include "hw/virtio/virtio.h"
> @@ -63,7 +64,7 @@ static void vhost_user_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>      /* Our num_queues overrides the device backend */
>      virtio_stw_p(vdev, &s->blkcfg.num_queues, s->num_queues);
>  
> -    memcpy(config, &s->blkcfg, sizeof(struct virtio_blk_config));
> +    memcpy(config, &s->blkcfg, s->config_size);

Please drop the s->config_size field introduced in this patch and use
vdev->config_len instead. When the same value is stored in multiple
places it's hard to be sure each copy remains identical and bugs can
creep in.

For example, if vdev->config_len is used consistently then it's clear
that buffer overflows and information leaks are prevented by common
code:

  uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr)
  {
      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
      uint8_t val;

      if (addr + sizeof(val) > vdev->config_len) {
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          return (uint32_t)-1;
      }

      k->get_config(vdev, vdev->config);

It's safe because vdev->config is g_malloc0(vdev->config_len).

Buf if I see s->config_size, I don't really know whether it's safe and I
need to audit the code to be sure.