linux-kernel - [PATCH] drm/v3d: Fix null pointer dereference of pointer perfmon

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Date:   Sun, 24 Apr 2022 19:35:12 +0100
From:   Colin Ian King <colin.i.king@...il.com>
To:     Emma Anholt <emma@...olt.net>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Melissa Wen <mwen@...lia.com>,
        "Juan A . Suarez Romero" <jasuarez@...lia.com>,
        dri-devel@...ts.freedesktop.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] drm/v3d: Fix null pointer dereference of pointer perfmon

In the unlikely event that pointer perfmon is null the WARN_ON return path
occurs after the pointer has already been deferenced. Fix this by only
dereferencing perfmon after it has been null checked.

Fixes: 26a4dc29b74a ("drm/v3d: Expose performance counters to userspace")
Signed-off-by: Colin Ian King <colin.i.king@...il.com>
---
 drivers/gpu/drm/v3d/v3d_perfmon.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/v3d/v3d_perfmon.c b/drivers/gpu/drm/v3d/v3d_perfmon.c
index 0288ef063513..f6a88abccc7d 100644
--- a/drivers/gpu/drm/v3d/v3d_perfmon.c
+++ b/drivers/gpu/drm/v3d/v3d_perfmon.c
@@ -25,11 +25,12 @@ void v3d_perfmon_start(struct v3d_dev *v3d, struct v3d_perfmon *perfmon)
 {
 	unsigned int i;
 	u32 mask;
-	u8 ncounters = perfmon->ncounters;
+	u8 ncounters;
 
 	if (WARN_ON_ONCE(!perfmon || v3d->active_perfmon))
 		return;
 
+	ncounters = perfmon->ncounters;
 	mask = GENMASK(ncounters - 1, 0);
 
 	for (i = 0; i < ncounters; i++) {
-- 
2.35.1