Re: [PATCH v2 2/3] virtio-gpu: fix hang under TCG when unmapping blob

["Alex Bennée" <alex.bennee@linaro.org>]

Manos Pitsidianakis <manos.pitsidianakis@linaro.org> writes:

> From: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
>
> 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
> thus no interrupt 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 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.
>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro