linux-kernel - [PATCH] drm/vc4: fix dereference before NULL check

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Date:   Tue, 28 Jun 2022 14:36:57 +0800
From:   Yu Liao <liaoyu15@...wei.com>
To:     <emma@...olt.net>, <mripard@...nel.org>, <eric@...olt.net>,
        <bbrezillon@...nel.org>
CC:     <liaoyu15@...wei.com>, <liwei391@...wei.com>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH] drm/vc4: fix dereference before NULL check

The "perfmon" pointer is equal to the return value of idr_find
which may be NULL, access by vc4_perfmon_get before checking if
it was NULL. Fix this by dereferencing "perfmon" after "perfmon"
has been null checked.

Addresses-Coverity: ("Dereference before null check")
Fixes: 65101d8c9108 ("drm/vc4: Expose performance counters to userspace")
Signed-off-by: Yu Liao <liaoyu15@...wei.com>
---
 drivers/gpu/drm/vc4/vc4_perfmon.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_perfmon.c b/drivers/gpu/drm/vc4/vc4_perfmon.c
index c7f5adb6bcf8..ca969b8c7dde 100644
--- a/drivers/gpu/drm/vc4/vc4_perfmon.c
+++ b/drivers/gpu/drm/vc4/vc4_perfmon.c
@@ -17,13 +17,17 @@
 
 void vc4_perfmon_get(struct vc4_perfmon *perfmon)
 {
-	struct vc4_dev *vc4 = perfmon->dev;
+	struct vc4_dev *vc4;
+
+	if (!perfmon)
+		return;
+
+	vc4 = perfmon->dev;
 
 	if (WARN_ON_ONCE(vc4->is_vc5))
 		return;
 
-	if (perfmon)
-		refcount_inc(&perfmon->refcnt);
+	refcount_inc(&perfmon->refcnt);
 }
 
 void vc4_perfmon_put(struct vc4_perfmon *perfmon)
-- 
2.25.1