[PATCH] device: fix missing check on list iterator

[Xiaomeng Tong <xiam0nd.tong@gmail.com>]

The bug is here:
	lo = pstate->base.domain[domain->name];

The list iterator 'pstate' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case should
be checked before any use of the iterator, otherwise it will lead
to a invalid memory access.

To fix this bug, add an check. Use a new value 'iter' as the list
iterator, while use the old value 'pstate' as a dedicated variable
to point to the found element.

Cc: stable@vger.kernel.org
Fixes: 9838366c1597d ("drm/nouveau/device: initial control object class, with pstate control methods")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
---
 drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
index ce774579c89d..6b768635e8ba 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
@@ -72,7 +72,7 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
 	} *args = data;
 	struct nvkm_clk *clk = ctrl->device->clk;
 	const struct nvkm_domain *domain;
-	struct nvkm_pstate *pstate;
+	struct nvkm_pstate *pstate = NULL, *iter;
 	struct nvkm_cstate *cstate;
 	int i = 0, j = -1;
 	u32 lo, hi;
@@ -103,11 +103,16 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
 		return -EINVAL;
 
 	if (args->v0.state != NVIF_CONTROL_PSTATE_ATTR_V0_STATE_CURRENT) {
-		list_for_each_entry(pstate, &clk->states, head) {
-			if (i++ == args->v0.state)
+		list_for_each_entry(iter, &clk->states, head) {
+			if (i++ == args->v0.state) {
+				pstate = iter;
 				break;
+			}
 		}
 
+		if (!pstate)
+			return -EINVAL;
+
 		lo = pstate->base.domain[domain->name];
 		hi = lo;
 		list_for_each_entry(cstate, &pstate->list, head) {
-- 
2.17.1