On 2026/07/28 19:58, [email protected] wrote:
From: Marc-André Lureau <[email protected]>
virtio_gpu_cleanup_mapping() tears down the udmabuf mapping (or, for the
small-iov case, the guest memory mapping) via virtio_gpu_fini_udmabuf(),
but leaves res->blob pointing at it afterwards. Once a blob resource
goes through RESOURCE_DETACH_BACKING, res->blob is left dangling while
res->blob_size is still non-zero.
virtio_gpu_update_cursor_data() only checks res->blob_size before
copying from res->blob, so UPDATE_CURSOR on a detached blob resource
makes QEMU memcpy from freed memory.
virtio_gpu_do_set_scanout() also creates scanout->ds over
res->blob + fb->offset. RESOURCE_DETACH_BACKING leaves that surface
installed, so a subsequent display refresh can still read the backing
after virtio_gpu_fini_udmabuf() has unmapped it.
Disable any active scanouts referencing the resource before tearing down
its backing, clear res->blob afterwards, and check it before
dereferencing in virtio_gpu_update_cursor_data().
Fixes: CVE-2026-66020
Fixes: bdd53f739273 ("virtio-gpu: Update cursor data using blob")
Fixes: 32db3c63ae11 ("virtio-gpu: Add virtio_gpu_set_scanout_blob")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3881
Reported-by: 章鱼哥@aipy (www.aipyaipy.com)
Reported-by: swing <[email protected]>
Signed-off-by: Marc-André Lureau <[email protected]>
---
v3:
- only disable blob-based scanouts on detach_backing
---
hw/display/virtio-gpu.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index 574e68992925..9bacd38b4521 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -63,8 +63,16 @@ void virtio_gpu_update_cursor_data(VirtIOGPU *g,
}
data = pixman_image_get_data(res->image);
} else {
+ if (!res->blob) {
+ qemu_log_mask(LOG_GUEST_ERROR, "%s: resource %d has no blob\n",
+ __func__, resource_id);
+ return;
+ }
if (res->blob_size < (s->current_cursor->width *
s->current_cursor->height * 4)) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "%s: blob size too small for resource %d\n",
+ __func__, resource_id);
return;
}
data = res->blob;
@@ -974,6 +982,7 @@ void virtio_gpu_cleanup_mapping(VirtIOGPU *g,
if (res->blob) {
virtio_gpu_fini_udmabuf(g, res);
+ res->blob = NULL;
}
}
@@ -1039,6 +1048,15 @@ virtio_gpu_resource_detach_backing(VirtIOGPU *g,
if (!res) {
return;
}
+
+ if (res->blob) {
+ for (int i = 0; i < g->parent_obj.conf.max_outputs; i++) {
+ if (res->scanout_bitmask & (1 << i)) {
+ virtio_gpu_disable_scanout(g, i);
virtio_gpu_disable_scanout() switches the software surface but does not
call qemu_console_gl_scanout_disable() and
qemu_console_gl_release_dmabuf(). Consequently, GTK/SDL can remain in
dmabuf scanout mode and continue displaying/pinning the imported guest
backing after QEMU considers it detached. virtio_gpu_fini_udmabuf() only
closes the fd.
Regards,
Akihiko Odaki
+ }
+ }
+ }
+
virtio_gpu_cleanup_mapping(g, res);
}
