linux-kernel - [PATCH v2 1/2] drm/nouveau/device: avoid usage of list iterator after loop

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Message-Id: <20230301-drm-nouveau-avoid-iter-after-loop-v2-1-1e6428cc7fa8@gmail.com>
Date:   Mon, 13 Mar 2023 14:54:49 +0100
From:   Jakob Koschel <jkl820.git@...il.com>
To:     Ben Skeggs <bskeggs@...hat.com>, Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Pietro Borrello <borrello@...g.uniroma1.it>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>, Jakob Koschel <jkl820.git@...il.com>
Subject: [PATCH v2 1/2] drm/nouveau/device: avoid usage of list iterator
 after loop

If potentially no valid element is found, 'pstate' would contain an
invalid pointer past the iterator loop. To ensure 'pstate' is always
valid, we only set it if the correct element was found. That allows
adding a WARN_ON() in case the code works incorrectly, exposing
currently undetectable potential bugs.

Additionally, Linus proposed to avoid any use of the list iterator
variable after the loop, in the attempt to move the list iterator
variable declaration into the macro to avoid any potential misuse after
the loop [1].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jkl820.git@...il.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..8ae14ab8f88e 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 (WARN_ON_ONCE(!pstate))
+			return -EINVAL;
+
 		lo = pstate->base.domain[domain->name];
 		hi = lo;
 		list_for_each_entry(cstate, &pstate->list, head) {

-- 
2.34.1