On 2025/04/30 3:48, Dmitry Osipenko wrote:
On 4/28/25 15:59, Alex Bennée wrote:
From: Manos Pitsidianakis <manos.pitsidiana...@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.
The RCU thread should be free to kick in if the guest is waiting and
idle. That may be a problem that should be analyzed and fixed.
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 <m...@redhat.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidiana...@linaro.org>
Reviewed-by: Alex Bennée <alex.ben...@linaro.org>
Tested-by: Alex Bennée <alex.ben...@linaro.org>
Message-Id: <20250410122643.1747913-3-manos.pitsidiana...@linaro.org>
Cc: qemu-sta...@nongnu.org
---
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 71a7500de9..8fbe4e70cc 100644
--- a/hw/display/virtio-gpu-virgl.c
+++ b/hw/display/virtio-gpu-virgl.c
@@ -112,7 +112,7 @@ virtio_gpu_virgl_map_resource_blob(VirtIOGPU *g,
vmr->g = g;
mr = g_new0(MemoryRegion, 1);
- 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);
This change makes QEMU to crash.
AFAICT, it effectively reverts code to old bugged version [1] that was
rejected in the past.
+Akihiko Odaki
[1]
https://lore.kernel.org/qemu-devel/20230915111130.24064-10-ray.hu...@amd.com/
I think you are right.
Changing the owner indeed makes the MR to be freed right away, but this
is because it makes a reference by the FlatView to the MR dangling and
can lead to use-after-free.
Setting the owner to the device implies that the device keeps the
storage of the MR and. The references to the MR must be counted by the
device to keep the storage available in such a case.
The MR itself doesn't count the references. It is fine as long as the MR
is kept alive by being parented by the device, but
virtio_gpu_virgl_unmap_resource_blob() unparents the MR and breaks the
assumption. That's why the device shouldn't be the the owner of the MR.
Regards,
Akihiko Odaki
