[PATCH v2] hw/display/virtio-gpu: fix use-after-free on blob detach backing

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_cleanup_mapping(g, res);
 }
 
-- 
2.55.0