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Tsirkin" Subject: [PATCH v2] hw/display/virtio-gpu: fix use-after-free on blob detach backing Date: Sat, 25 Jul 2026 15:37:27 +0400 Message-ID: <20260725113727.1692271-1-marcandre.lureau@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=marcandre.lureau@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -1 X-Spam_score: -0.2 X-Spam_bar: / X-Spam_report: (-0.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.419, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1784979491585154100 From: Marc-Andr=C3=A9 Lureau 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") Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3881 Reported-by: =E7=AB=A0=E9=B1=BC=E5=93=A5@aipy (www.aipyaipy.com) Reported-by: swing Signed-off-by: Marc-Andr=C3=A9 Lureau --- v2: - disable active scanouts referencing the resource before tearing down its backing, fixing a second use-after-free through the display refresh path - add LOG_GUEST_ERROR for the blob-size-too-small early return --- 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 211a529ed720..b21ac8dace08 100644 --- a/hw/display/virtio-gpu.c +++ b/hw/display/virtio-gpu.c @@ -57,8 +57,16 @@ void virtio_gpu_update_cursor_data(VirtIOGPU *g, } =20 if (res->blob_size) { + 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 =3D res->blob; @@ -969,6 +977,7 @@ void virtio_gpu_cleanup_mapping(VirtIOGPU *g, =20 if (res->blob) { virtio_gpu_fini_udmabuf(g, res); + res->blob =3D NULL; } } =20 @@ -1031,6 +1040,15 @@ virtio_gpu_resource_detach_backing(VirtIOGPU *g, if (!res) { return; } + + if (res->scanout_bitmask) { + for (int i =3D 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); } =20 --=20 2.55.0