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

["Cédric Le Goater" <clg@redhat.com>]

On 2/19/25 21:34, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@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.szmigiero@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;
>