[PATCH 1/2] drm/nouveau/sw: prevent NULL deref of disp in vblank methods

[Zhenhao Wan]

The NV50 and GF100 software-class vblank methods nv50_sw_chan_mthd()
and gf100_sw_chan_mthd() evaluate "data < device->disp->vblank.index_nr"
to validate the requested vblank head before allowing its notifier.

On a headless card the display subdevice is absent: nvkm_subdev_disable()
(strap-driven from devinit) and the -ENODEV path of the NVKM_LAYOUT_ONCE
device constructor both leave device->disp == NULL while the device
continues to probe and still registers a render node (DRIVER_RENDER). An
unprivileged client holding a /dev/dri/renderD* fd can then create a
software channel and push method 0x0408 (NV50) / 0x040c (GF100), which
dereferences the NULL device->disp and oopses the kernel.

The channel constructors nv50_sw_chan_new() and gf100_sw_chan_new()
already tolerate an absent display ("for (i = 0; disp && ...)"), so no
vblank notifier is ever registered on such a card and the method handlers
are the only path that assumes disp is present. Guard the dereference
with the same disp NULL test the constructors already use.

Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Assisted-by: Claude:claude-opus-5
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhenhao Wan <whi4ed0g@xxxxxxxxx>
---
 drivers/gpu/drm/nouveau/nvkm/engine/sw/gf100.c | 2 +-
 drivers/gpu/drm/nouveau/nvkm/engine/sw/nv50.c  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/sw/gf100.c b/drivers/gpu/drm/nouveau/nvkm/engine/sw/gf100.c
index 0171cdf6f639..4cf8cd120c76 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/sw/gf100.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/sw/gf100.c
@@ -72,7 +72,7 @@ gf100_sw_chan_mthd(struct nvkm_sw_chan *base, int subc, u32 mthd, u32 data)
 		chan->vblank.value = data;
 		return true;
 	case 0x040c:
-		if (data < device->disp->vblank.index_nr) {
+		if (device->disp && data < device->disp->vblank.index_nr) {
 			nvkm_event_ntfy_allow(&chan->vblank.notify[data]);
 			return true;
 		}
diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/sw/nv50.c b/drivers/gpu/drm/nouveau/nvkm/engine/sw/nv50.c
index 0cfb1eaae6de..b202e11238a5 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/sw/nv50.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/sw/nv50.c
@@ -69,7 +69,7 @@ nv50_sw_chan_mthd(struct nvkm_sw_chan *base, int subc, u32 mthd, u32 data)
 	case 0x0400: chan->vblank.offset = data; return true;
 	case 0x0404: chan->vblank.value  = data; return true;
 	case 0x0408:
-		if (data < device->disp->vblank.index_nr) {
+		if (device->disp && data < device->disp->vblank.index_nr) {
 			nvkm_event_ntfy_allow(&chan->vblank.notify[data]);
 			return true;
 		}

-- 
2.34.1