[PATCH v2 2/2] drm/nouveau/kms: guard NULL crtc in nv50_sor_atomic_disable()

[Marek Czernohous <mczernohous@gmail.com>]

From: Marek Czernohous <mczernohous@gmail.com>

From: Marek Czernohous <marek@czernohous.de>

nv50_sor_atomic_disable() unconditionally computes
nv50_head(nv_encoder->crtc) and dereferences the result a few lines
later.  nv_encoder->crtc is nouveau's own shadow pointer, set in
.atomic_enable and cleared at the end of .atomic_disable.
Commit f575f2bdb6c3 ("drm/nouveau/kms/nv50-: Remove (nv_encoder->crtc)
checks in ->disable callbacks") removed the NULL check here,
reasoning that the atomic helpers never call ->disable without a
crtc.  On NVAC (MCP79) under
Wayland sessions (observed with Weston's DRM backend and with
labwc/wlroots) we have hit the NULL case in practice during session
teardown and VT switches: disable runs without (or after) its matching
enable, and because nv50_head() is container_of(), the NULL does not
stay NULL but becomes a small bogus pointer, so the subsequent head
dereferences fault and the kernel oopses.

Restore the guard, as drm_WARN_ON_ONCE() instead of a silent return:
a NULL crtc here still indicates a state-tracking inconsistency that
should stay visible.  Return without touching the output; in this path
either enable never ran (nothing to tear down) or an earlier disable
already did the teardown, and the encoder release is handled by the
commit_tail release loop in both cases.

Fixes: f575f2bdb6c3 ("drm/nouveau/kms/nv50-: Remove (nv_encoder->crtc) checks in ->disable callbacks")
Cc: <stable@vger.kernel.org>
Tested-by: Fab Stz <fabstz-it@yahoo.fr>
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Marek Czernohous <marek@czernohous.de>
---
v1 -> v2: dropped the nvif_outp_release() call from the early return
(the release is owned by the commit_tail release loop; calling it here
released twice and detached the OR before the disable flush); turned
the silent return into drm_WARN_ON_ONCE(); reworded the commit message
(the v1 "race between atomic_check and atomic_commit" wording did not
match the mechanism).
v1: https://lore.kernel.org/nouveau/20260409172126.115441-3-marek@czernohous.de/

 drivers/gpu/drm/nouveau/dispnv50/disp.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c b/drivers/gpu/drm/nouveau/dispnv50/disp.c
index 6c3a871..597bc64 100644
--- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
+++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
@@ -1565,14 +1565,28 @@ static void
 nv50_sor_atomic_disable(struct drm_encoder *encoder, struct drm_atomic_state *state)
 {
 	struct nouveau_encoder *nv_encoder = nouveau_encoder(encoder);
-	struct nv50_head *head = nv50_head(nv_encoder->crtc);
+	struct nv50_head *head;
 #ifdef CONFIG_DRM_NOUVEAU_BACKLIGHT
 	struct nouveau_connector *nv_connector = nv50_outp_get_old_connector(state, nv_encoder);
 	struct nouveau_drm *drm = nouveau_drm(nv_encoder->base.base.dev);
 	struct nouveau_backlight *backlight = nv_connector->backlight;
 	struct drm_dp_aux *aux = &nv_connector->aux;
 	int ret;
+#endif
+
+	/* nv_encoder->crtc is the driver's shadow pointer, set in
+	 * .atomic_enable and cleared at the end of this function.  NULL here
+	 * means disable-without-enable or a double disable; bail before
+	 * container_of() turns it into a bogus head pointer (checking the
+	 * result would not work, container_of(NULL) is never NULL).  The
+	 * encoder release is handled by the commit_tail release loop, so
+	 * there is nothing to clean up here.
+	 */
+	if (drm_WARN_ON_ONCE(encoder->dev, !nv_encoder->crtc))
+		return;
+	head = nv50_head(nv_encoder->crtc);
 
+#ifdef CONFIG_DRM_NOUVEAU_BACKLIGHT
 	if (backlight && backlight->uses_dpcd) {
 		ret = drm_edp_backlight_disable(aux, &backlight->edp_info);
 		if (ret < 0)
-- 
2.53.0