CVE-2026-89803: drm/nouveau: unsubscribe the channel-kill event before the fence context

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

drm/nouveau: unsubscribe the channel-kill event before the fence context

nouveauchanneldel() tears the fence context down first and only drops the channel-kill subscription later, in the middle of the nvif object teardown:

if (chan->fence) nouveaufence(chan->cli->drm)->contextdel(chan); ... nvifobjectdtor(&chan->vram); nvifeventdtor(&chan->kill);

The subscribed handler is nouveauchannelkilled(), which calls nouveauchannelkill() and from there nouveaufencecontextkill() on chan->fence. A kill event delivered in that window takes fctx->lock and walks fctx->pending on a fence context that contextdel() has already freed.

Nothing reaches this below Fermi today, because the subscription is gated on FERMICHANNELGPFIFO and nothing kills a channel there. On Fermi and newer the window is real but narrow, since a kill has to land exactly while the channel is being destroyed. That is reason enough on its own, which is why this carries a Fixes: tag. The last patch in this series subscribes Tesla channels as well; nothing kills those today, so it does not widen the exposure now, but it is the groundwork for a recovery path that would, and the ordering is better fixed before that lands than alongside it.

Drop the subscription before anything it depends on is torn down.