linux-kernel - re: watchdog: Add hrtimer-based pretimeout feature

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Date:   Tue, 22 Jun 2021 16:27:34 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Curtis Klein <curtis.klein@....com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-watchdog@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: watchdog: Add hrtimer-based pretimeout feature

Hi,

Static analysis using Coverity on linux-next has found an issue in
function watchdog_cdev_unregister in source
drivers/watchdog/watchdog_dev.c with the following commit:

commit 7b7d2fdc8c3e3f9fdb3558d674e1eeddc16c7d9e
Author: Curtis Klein <curtis.klein@....com>
Date:   Wed Feb 3 12:11:30 2021 -0800

    watchdog: Add hrtimer-based pretimeout feature

The analysis is as follows:

1084 static void (struct watchdog_device *wdd)
1085 {
1086        struct watchdog_core_data *wd_data = wdd->wd_data;
1087
1088        cdev_device_del(&wd_data->cdev, &wd_data->dev);

    1. Condition wdd->id == 0, taking true branch.

1089        if (wdd->id == 0) {
1090                misc_deregister(&watchdog_miscdev);
1091                old_wd_data = NULL;
1092        }
1093

    2. Condition watchdog_active(wdd), taking true branch.
    3. Condition test_bit(4, &wdd->status), taking true branch.

1094        if (watchdog_active(wdd) &&
1095            test_bit(WDOG_STOP_ON_UNREGISTER, &wdd->status)) {
1096                watchdog_stop(wdd);
1097        }
1098
1099        mutex_lock(&wd_data->lock);
1100        wd_data->wdd = NULL;

    4. assign_zero: Assigning: wdd->wd_data = NULL.

1101        wdd->wd_data = NULL;
1102        mutex_unlock(&wd_data->lock);
1103
1104        hrtimer_cancel(&wd_data->timer);
1105        kthread_cancel_work_sync(&wd_data->work);

Explicit null dereferenced (FORWARD_NULL)

    5. var_deref_model: Passing wdd to watchdog_hrtimer_pretimeout_stop,
which dereferences null wdd->wd_data.

1106        watchdog_hrtimer_pretimeout_stop(wdd);
1107
1108        put_device(&wd_data->dev);
1109 }

The call to watchdog_hrtimer_pretimeout_stop dereferences wdd as follows:

41 void watchdog_hrtimer_pretimeout_stop(struct watchdog_device *wdd)
42 {

  1. deref_parm_field_in_call: Function hrtimer_cancel dereferences an
offset off wdd->wd_data.

43        hrtimer_cancel(&wdd->wd_data->pretimeout_timer);
44 }

Since wdd->wd_data is set to NULL on line 1101, the call to
watchdog_hrtimer_pretimeout_stop will always trip a null pointer
dereference.  Shall we just remove that call?

Colin