Re: [PATCH v1 1/2] virtio-gpu: fix hang under TCG when unmapping blob

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 03, 2025 at 03:17:01PM +0300, Manos Pitsidianakis wrote:
> This commit fixes an indefinite hang when using VIRTIO GPU blob objects
> under TCG in certain conditions.
> 
> The VIRTIO_GPU_CMD_RESOURCE_MAP_BLOB VIRTIO command creates a
> MemoryRegion and attaches it to an offset on a PCI BAR of the
> VirtIOGPUdevice. The VIRTIO_GPU_CMD_RESOURCE_UNMAP_BLOB command unmaps
> it.
> 
> Because virglrenderer commands are not thread-safe they are only
> called on the main context and QEMU performs the cleanup in three steps
> to prevent a use-after-free scenario where the guest can access the
> region after it’s unmapped:
> 
> 1. From the main context, the region’s field finish_unmapping is false
>    by default, so it sets a variable cmd_suspended, increases the
>    renderer_blocked variable, deletes the blob subregion, and unparents
>    the blob subregion causing its reference count to decrement.
> 
> 2. From an RCU context, the MemoryView gets freed, the FlatView gets
>    recalculated, the free callback of the blob region
>    virtio_gpu_virgl_hostmem_region_free is called which sets the
>    region’s field finish_unmapping to true, allowing the main thread
>    context to finish replying to the command
> 
> 3. From the main context, the command is processed again, but this time
>    finish_unmapping is true, so virgl_renderer_resource_unmap can be
>    called and a response is sent to the guest.
> 
> It happens so that under TCG, if the guest has no timers configured (and
> has no other interrupts that will cause the CPU to exit), the RCU thread
> does not have enough time to grab the locks and recalculate the
> FlatView.
> 
> That’s not a big problem in practice since most guests will assume a
> response will happen later in time and go on to do different things,
> potentially triggering interrupts and allowing the RCU context to run.
> If the guest waits for the unmap command to complete though, it blocks
> indefinitely. Attaching to the QEMU monitor and force quitting the guest
> allows the cleanup to continue.
> 
> There's no reason why the FlatView recalculation can't occur right away
> when we delete the blob subregion, however. It does not, because when we
> create the subregion we set the object as its own parent:
> 
>     memory_region_init_ram_ptr(mr, OBJECT(mr), "blob", size, data);
> 
> The extra self-reference is what prevents freeing the memory region
> object in the memory transaction of deleting the subregion.
> 
> This commit changes the owner object to the device, which removes the
> extra owner reference in the memory region and causes the MR to be
> freed right away in the main context.
> 
> Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  hw/display/virtio-gpu-virgl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/display/virtio-gpu-virgl.c b/hw/display/virtio-gpu-virgl.c
> index 145a0b3879..ad600ccc9c 100644
> --- a/hw/display/virtio-gpu-virgl.c
> +++ b/hw/display/virtio-gpu-virgl.c
> @@ -120,7 +120,7 @@ virtio_gpu_virgl_map_resource_blob(VirtIOGPU *g,
>      vmr->g = g;
>  
>      mr = &vmr->mr;
> -    memory_region_init_ram_ptr(mr, OBJECT(mr), "blob", size, data);
> +    memory_region_init_ram_ptr(mr, OBJECT(g), "blob", size, data);
>      memory_region_add_subregion(&b->hostmem, offset, mr);
>      memory_region_set_enabled(mr, true);
>  
> -- 
> γαῖα πυρί μιχθήτω