linux-kernel - [PATCH v2] opp: use list iterator only inside the loop

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Message-Id: <20220331023608.30497-1-xiam0nd.tong@gmail.com>
Date:   Thu, 31 Mar 2022 10:36:08 +0800
From:   Xiaomeng Tong <xiam0nd.tong@...il.com>
To:     vireshk@...nel.org, nm@...com, sboyd@...nel.org
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xiaomeng Tong <xiam0nd.tong@...il.com>
Subject: [PATCH v2] opp: use list iterator only inside the loop

The list iterator 'new_dev' will point to a bogus position containing
HEAD if any one of these conditions is possible: the list is empty or
no element is found, thus can potentially lead to an invalid memory
access in 'dev = new_dev->dev;'.

As discussed before, we should avoid to use a list iterator variable
outside the loop which is considered harmful[1].

In this case, use a new variable 'iter' as the list iterator, while
use the old variable 'new_dev' as a dedicated pointer to point to the
found entry. And BUG_ON(!new_dev);.

[1]:  https://lkml.org/lkml/2022/2/17/1032

Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
---

changes since v1:
 - use BUG_ON(!new_dev); instead of return; (Viresh Kumar)

v1: https://lore.kernel.org/lkml/20220331015818.28045-1-xiam0nd.tong@gmail.com/

---
 drivers/opp/debugfs.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/opp/debugfs.c b/drivers/opp/debugfs.c
index 596c185b5dda..81b2bc4b5f43 100644
--- a/drivers/opp/debugfs.c
+++ b/drivers/opp/debugfs.c
@@ -187,14 +187,18 @@ void opp_debug_register(struct opp_device *opp_dev, struct opp_table *opp_table)
 static void opp_migrate_dentry(struct opp_device *opp_dev,
 			       struct opp_table *opp_table)
 {
-	struct opp_device *new_dev;
+	struct opp_device *new_dev = NULL, *iter;
 	const struct device *dev;
 	struct dentry *dentry;
 
 	/* Look for next opp-dev */
-	list_for_each_entry(new_dev, &opp_table->dev_list, node)
-		if (new_dev != opp_dev)
+	list_for_each_entry(iter, &opp_table->dev_list, node)
+		if (iter != opp_dev) {
+			new_dev = iter;
 			break;
+		}
+
+	BUG_ON(!new_dev);
 
 	/* new_dev is guaranteed to be valid here */
 	dev = new_dev->dev;
-- 
2.17.1