[PATCH 5.15.y 2/2] drm/nouveau: unsubscribe the channel-kill event before the fence context

[Sasha Levin <sashal@kernel.org>]

From: Marek Czernohous <marek@czernohous.de>

[ Upstream commit 511585987d27d8cb668acebd399fc4deda23404c ]

nouveau_channel_del() 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)
		nouveau_fence(chan->cli->drm)->context_del(chan);
	...
	nvif_object_dtor(&chan->vram);
	nvif_event_dtor(&chan->kill);

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

Nothing reaches this below Fermi today, because the subscription is
gated on FERMI_CHANNEL_GPFIFO 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.

Fixes: ea13e5abf807 ("drm/nouveau: signal pending fences when channel has been killed")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Marek Czernohous <marek@czernohous.de>
Fixes: ea13e5abf807 ("drm/nouveau: signal pending fences when channel has been killed")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260812231330.705425-2-mczernohous@gmail.com
[ replaced nvif_event_dtor() with the older nvif_notify_dtor() API. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/nouveau/nouveau_chan.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_chan.c b/drivers/gpu/drm/nouveau/nouveau_chan.c
index da345b50769cf..baba2f4509496 100644
--- a/drivers/gpu/drm/nouveau/nouveau_chan.c
+++ b/drivers/gpu/drm/nouveau/nouveau_chan.c
@@ -87,6 +87,14 @@ nouveau_channel_del(struct nouveau_channel **pchan)
 	if (chan) {
 		struct nouveau_cli *cli = (void *)chan->user.client;
 
+		/*
+		 * Drop the kill-event subscription first.  Its handler
+		 * dereferences chan->fence, which the fence context teardown
+		 * below frees, so leaving it armed across the teardown leaves
+		 * a window for a use-after-free.
+		 */
+		nvif_notify_dtor(&chan->kill);
+
 		if (chan->fence)
 			nouveau_fence(chan->drm)->context_del(chan);
 
@@ -96,7 +104,6 @@ nouveau_channel_del(struct nouveau_channel **pchan)
 		nvif_object_dtor(&chan->nvsw);
 		nvif_object_dtor(&chan->gart);
 		nvif_object_dtor(&chan->vram);
-		nvif_notify_dtor(&chan->kill);
 		nvif_object_dtor(&chan->user);
 		nvif_object_dtor(&chan->push.ctxdma);
 		nouveau_vma_del(&chan->push.vma);
-- 
2.53.0