[PATCH 3/3] drm/nouveau/gsp: Fix NULL dereference in nvkm_gsp_gcx_ready()

[Jim Cromie via B4 Relay <devnull+jim.cromie.gmail.com@kernel.org>]

From: Jim Cromie <jim.cromie@gmail.com>

Commit cb4c7603678c ("drm/nouveau/gsp/r570: Add support for
INTERNAL_GCX_ENTRY_PREREQUISITE") hooked nvif_device_gcx_ready() into
nouveau_pmops_runtime_suspend() to consult GSP before runtime suspend.

On GPUs running without GSP-RM (e.g. Volta GV100, NvGspRm=0, or missing
GSP firmware blobs), device->gsp is instantiated but gsp->rm remains
NULL. nvkm_udevice_gcx_ready() checked "!gsp" instead of verifying
nvkm_gsp_rm(gsp), allowing non-RM instances to invoke
nvkm_gsp_gcx_ready(). nvkm_gsp_gcx_ready() then unconditionally
dereferenced gsp->rm->api, causing a kernel oops:

  RIP: 0010:nvkm_gsp_gcx_ready+0x10/0x30 [nouveau]
  Code: ... 48 8b 87 70 0e 00 00 <48> 8b 40 18 ...
  RAX: 0000000000000000 RDI: ffff8c9555fca000
  Call Trace:
   nvkm_udevice_mthd+0xa5/0x150 [nouveau]
   nvkm_ioctl+0xcc/0x1d0 [nouveau]
   nvif_object_mthd+0x110/0x1e0 [nouveau]
   nvif_device_gcx_ready+0x36/0x60 [nouveau]
   nouveau_pmops_runtime_suspend+0x39/0x140 [nouveau]

Instruction decode shows `mov rax, [rdi+0xe70]` loads gsp->rm (0x0),
and `<48> 8b 40 18` (`mov rax, [rax+0x18]`) faults accessing rm->api.

Fix by:
0. Checking !nvkm_gsp_rm(gsp) in nvkm_udevice_gcx_ready() so non-RM
   devices report GC6/GcOff ready and bypass the GSP call entirely.
1. Guarding gsp and gsp->rm in nvkm_gsp_gcx_ready() before inspecting
   gsp->rm->api->gsp->gcx_ready.

Fixes: cb4c7603678c ("drm/nouveau/gsp/r570: Add support for INTERNAL_GCX_ENTRY_PREREQUISITE")
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
 drivers/gpu/drm/nouveau/nvkm/engine/device/user.c | 2 +-
 drivers/gpu/drm/nouveau/nvkm/subdev/gsp/base.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/user.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/user.c
index f78e6b9b4292..d469437eb991 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/device/user.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/user.c
@@ -201,7 +201,7 @@ nvkm_udevice_gcx_ready(struct nvkm_udevice *udev, void *data, u32 size)
 	} *args = data;
 	int ret = -ENOSYS;
 
-	if (!gsp) {
+	if (!nvkm_gsp_rm(gsp)) {
 		args->v0.ready = NV_DEVICE_GC6_READY | NV_DEVICE_GCOFF_READY;
 		return 0;
 	}
diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/base.c b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/base.c
index e475d0e8fa7b..9e07d67dcb0f 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/base.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/base.c
@@ -51,7 +51,7 @@ nvkm_gsp_intr_stall(struct nvkm_gsp *gsp, enum nvkm_subdev_type type, int inst)
 int
 nvkm_gsp_gcx_ready(struct nvkm_gsp *gsp)
 {
-	if (!gsp->rm->api->gsp->gcx_ready)
+	if (!gsp || !gsp->rm || !gsp->rm->api->gsp->gcx_ready)
 		return NV_DEVICE_GC6_READY | NV_DEVICE_GCOFF_READY;
 
 	return gsp->rm->api->gsp->gcx_ready(gsp);

-- 
2.55.0