drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip It's possible that mtk_crtc->event is NULL in mtk_drm_crtc_finish_page_flip(). pending_needs_vblank value is set by mtk_crtc->event, but in mtk_drm_crtc_atomic_flush(), it's is not guarded by the same lock in mtk_drm_finish_page_flip(), thus a race condition happens. Consider the following case: CPU1 CPU2 step 1: mtk_drm_crtc_atomic_begin() mtk_crtc->event is not null, step 1: mtk_drm_crtc_atomic_flush: mtk_drm_crtc_update_config( !!mtk_crtc->event) step 2: mtk_crtc_ddp_irq -> mtk_drm_finish_page_flip: lock mtk_crtc->event set to null, pending_needs_vblank set to false unlock pending_needs_vblank set to true, step 2: mtk_crtc_ddp_irq -> mtk_drm_finish_page_flip called again, pending_needs_vblank is still true //null pointer Instead of guarding the entire mtk_drm_crtc_atomic_flush(), it's more efficient to just check if mtk_crtc->event is null before use.

References

• https://git.kernel.org/stable/c/accdac6b71d5a2b84040c3d2234f53a60edc398e
• https://git.kernel.org/stable/c/dfde84cc6c589f2a9f820f12426d97365670b731
• https://git.kernel.org/stable/c/4688be96d20ffa49d2186523ee84f475f316fd49
• https://git.kernel.org/stable/c/9beec711a17245b853d64488fd5b739031612340
• https://git.kernel.org/stable/c/d2bd30c710475b2e29288827d2c91f9e6e2b91d7
• https://git.kernel.org/stable/c/a3dd12b64ae8373a41a216a0b621df224210860a
• https://git.kernel.org/stable/c/9acee29a38b4d4b70f1f583e5ef9a245db4db710
• https://git.kernel.org/stable/c/3fc88b246a2fc16014e374040fc15af1d3752535
• https://git.kernel.org/stable/c/c958e86e9cc1b48cac004a6e245154dfba8e163b