[PATCH 5.15 255/667] drm/v3d: Fix null pointer dereference of pointer perfmon

[Greg Kroah-Hartman]

From: Colin Ian King <colin.i.king@xxxxxxxxx>

[ Upstream commit ce7a1ecf3f9f1fccaf67295307614511d8e11b13 ]

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@xxxxxxxxx>
Reviewed-by: Juan A. Suarez <jasuarez@xxxxxxxxxx>
Signed-off-by: Melissa Wen <melissa.srw@xxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424183512.1365683-1-colin.i.king@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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