Re: [PATCH v5 34/36] vfio/migration: Max in-flight VFIO device state buffer count limit

[Cédric Le Goater]

On 2/19/25 21:34, Maciej S. Szmigiero wrote:

From: "Maciej S. Szmigiero" <maciej.szmigi...@oracle.com>

Allow capping the maximum count of in-flight VFIO device state buffers
queued at the destination, otherwise a malicious QEMU source could
theoretically cause the target QEMU to allocate unlimited amounts of memory
for buffers-in-flight.

Since this is not expected to be a realistic threat in most of VFIO live
migration use cases and the right value depends on the particular setup
disable the limit by default by setting it to UINT64_MAX.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigi...@oracle.com>
---
  hw/vfio/migration-multifd.c   | 14 ++++++++++++++
  hw/vfio/pci.c                 |  2 ++
  include/hw/vfio/vfio-common.h |  1 +
  3 files changed, 17 insertions(+)

diff --git a/hw/vfio/migration-multifd.c b/hw/vfio/migration-multifd.c
index 18a5ff964a37..04aa3f4a6596 100644
--- a/hw/vfio/migration-multifd.c
+++ b/hw/vfio/migration-multifd.c
@@ -53,6 +53,7 @@ typedef struct VFIOMultifd {
      QemuMutex load_bufs_mutex; /* Lock order: this lock -> BQL */
      uint32_t load_buf_idx;
      uint32_t load_buf_idx_last;
+    uint32_t load_buf_queued_pending_buffers;
  } VFIOMultifd;

static void vfio_state_buffer_clear(gpointer data)

@@ -121,6 +122,15 @@ static bool vfio_load_state_buffer_insert(VFIODevice 
*vbasedev,

assert(packet->idx >= multifd->load_buf_idx); + multifd->load_buf_queued_pending_buffers++;

+    if (multifd->load_buf_queued_pending_buffers >
+        vbasedev->migration_max_queued_buffers) {
+        error_setg(errp,
+                   "queuing state buffer %" PRIu32 " would exceed the max of 
%" PRIu64,
+                   packet->idx, vbasedev->migration_max_queued_buffers);
+        return false;
+    }
+
      lb->data = g_memdup2(&packet->data, packet_total_size - sizeof(*packet));
      lb->len = packet_total_size - sizeof(*packet);
      lb->is_present = true;
@@ -374,6 +384,9 @@ static bool vfio_load_bufs_thread(void *opaque, bool 
*should_quit, Error **errp)
              goto ret_signal;
          }

+ assert(multifd->load_buf_queued_pending_buffers > 0);

+        multifd->load_buf_queued_pending_buffers--;
+
          if (multifd->load_buf_idx == multifd->load_buf_idx_last - 1) {
              trace_vfio_load_state_device_buffer_end(vbasedev->name);
          }
@@ -408,6 +421,7 @@ VFIOMultifd *vfio_multifd_new(void)

multifd->load_buf_idx = 0;

      multifd->load_buf_idx_last = UINT32_MAX;
+    multifd->load_buf_queued_pending_buffers = 0;
      qemu_cond_init(&multifd->load_bufs_buffer_ready_cond);

multifd->load_bufs_thread_running = false;

diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 9111805ae06c..247418f0fce2 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -3383,6 +3383,8 @@ static const Property vfio_pci_dev_properties[] = {
                  vbasedev.migration_multifd_transfer,
                  qdev_prop_on_off_auto_mutable, OnOffAuto,
                  .set_default = true, .defval.i = ON_OFF_AUTO_AUTO),
+    DEFINE_PROP_UINT64("x-migration-max-queued-buffers", VFIOPCIDevice,
+                       vbasedev.migration_max_queued_buffers, UINT64_MAX),

UINT64_MAX doesn't make sense to me. What would be a reasonable value ?

Have you monitored the max ? Should we collect some statistics on this
value and raise a warning if a high water mark is reached ? I think
this would more useful.

      DEFINE_PROP_BOOL("migration-events", VFIOPCIDevice,
                       vbasedev.migration_events, false),
      DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),

Please add property documentation in vfio_pci_dev_class_init()


Thanks,

C.

diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
index 3006931accf6..30a5bb9af61b 100644
--- a/include/hw/vfio/vfio-common.h
+++ b/include/hw/vfio/vfio-common.h
@@ -155,6 +155,7 @@ typedef struct VFIODevice {
      bool ram_block_discard_allowed;
      OnOffAuto enable_migration;
      OnOffAuto migration_multifd_transfer;
+    uint64_t migration_max_queued_buffers;
      bool migration_events;
      VFIODeviceOps *ops;
      unsigned int num_irqs;