UBUNTU-CVE-2026-98156: Low severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: drm/virtio: use the DMA API for resource backing on Xen On a Xen PV domain page addresses bear no relation to the real machine addresses the host would have to use to reach it. virtioring.c handles this correctly, vringusemapapi() returns true for any xendomain() regardless of VIRTIOFACCESSPLATFORM. virtio-gpu makes the same decision independently, but its copy looks only at the feature bit: bool usedmaapi = !virtiohasdmaquirk(vgdev->vdev); QEMU does not set iommuplatform on virtio-vga by default, so VIRTIOFACCESSPLATFORM is not negotiated, usedmaapi is false, and virtiogpuobjectshmeminit() describes the framebuffer's backing pages to the host with sgphys(). Those are guest-physical addresses. In a PV domain they resolve, on the host side, to pages belonging to some other domain, so the host scans out unrelated memory. Move the decision into virtiogpuusedmaapi() and give it the xendomain() check, like vringusemapapi() has. This additionally enables the dmasyncsgtablefordevice() calls in virtgpuvq.c, which are required for correctness whenever swiotlb is in play. Reproduced with a Xen 4.21 PV dom0 nested inside QEMU 8.2 with virtio-vga, on both a distro 6.8 kernel and 6.18 LTS. A PVH dom0 works fine and doesn't need this fix because it is identity-mapped, only PV dom0s are affected.