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

Commit Message

Manos Pitsidianakis April 3, 2025, 12:17 p.m. UTC

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 <[e-mail]>
---
 hw/display/virtio-gpu-virgl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

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);