CVE-2024-26874

In the Linux kernel, the following vulnerability has been resolved:

drm/mediatek: Fix a null pointer crash in mtkdrmcrtcfinishpage_flip

It's possible that mtkcrtc->event is NULL in mtkdrmcrtcfinishpageflip().

pendingneedsvblank value is set by mtkcrtc->event, but in mtkdrmcrtcatomicflush(), it's is not guarded by the same lock in mtkdrmfinishpage_flip(), thus a race condition happens.

Consider the following case:

CPU1 CPU2 step 1: mtkdrmcrtcatomicbegin() mtkcrtc->event is not null, step 1: mtkdrmcrtcatomicflush: mtkdrmcrtcupdateconfig( !!mtkcrtc->event) step 2: mtkcrtcddpirq -> mtkdrmfinishpageflip: lock mtkcrtc->event set to null, pendingneedsvblank set to false unlock pendingneedsvblank 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 mtkdrmcrtcatomicflush(), it's more efficient to just check if mtk_crtc->event is null before use.