linux-kernel - [PATCH] usb: host: u132-hcd: Traverse u132_static_list under mutex lock in u132_hcd

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Date:   Thu,  2 Apr 2020 00:47:35 +0530
From:   madhuparnabhowmik10@...il.com
To:     gregkh@...uxfoundation.org, hariprasad.kelam@...il.com,
        colin.king@...onical.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org, andrianov@...ras.ru,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
Subject: [PATCH] usb: host: u132-hcd: Traverse u132_static_list under mutex lock in u132_hcd_exit

From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>

The global list u132_static_list is protected by u132_module_lock.
Elements are added to this list in the probe function and this list is
traversed in u132_hcd_exit() to unregister devices.

If probe and exit execute simultaneously there can be a race condition
between writing to this list in probe and reading the list in exit as
u132_module_lock is not held in exit function.

Even though u132_exiting variable is used in probe to detect if the module is
exiting, it is ineffective as the probe function may read the value
before it is updated in exit and thus leading to a race condition.

Therefore, hold u132_module_lock while traversing u132_static_list in
exit function.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
---
 drivers/usb/host/u132-hcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/u132-hcd.c b/drivers/usb/host/u132-hcd.c
index e9209e3e6248..1cadc4e0c9b2 100644
--- a/drivers/usb/host/u132-hcd.c
+++ b/drivers/usb/host/u132-hcd.c
@@ -3217,10 +3217,10 @@ static void __exit u132_hcd_exit(void)
 	struct u132 *temp;
 	mutex_lock(&u132_module_lock);
 	u132_exiting += 1;
-	mutex_unlock(&u132_module_lock);
 	list_for_each_entry_safe(u132, temp, &u132_static_list, u132_list) {
 		platform_device_unregister(u132->platform_dev);
 	}
+	mutex_unlock(&u132_module_lock);
 	platform_driver_unregister(&u132_platform_driver);
 	printk(KERN_INFO "u132-hcd driver deregistered\n");
 	wait_event(u132_hcd_wait, u132_instances == 0);
-- 
2.17.1