linux-kernel - Re: [PATCH -next] watchdog: Fix possible use-after-free by calling del_timer

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Date:   Wed, 12 May 2021 07:06:36 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Zou Wei <zou_wei@...wei.com>
Cc:     wim@...ux-watchdog.org, vz@...ia.com,
        linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] watchdog: Fix possible use-after-free by calling
 del_timer_sync()

On Wed, May 12, 2021 at 02:57:56PM +0800, Zou Wei wrote:
> This driver's remove path calls del_timer(). However, that function
> does not wait until the timer handler finishes. This means that the
> timer handler may still be running after the driver's remove function
> has finished, which would result in a use-after-free.
> 
> Fix by calling del_timer_sync(), which makes sure the timer handler
> has finished, and unable to re-schedule itself.
> 
> Reported-by: Hulk Robot <hulkci@...wei.com>
> Signed-off-by: Zou Wei <zou_wei@...wei.com>

If you have more of those, _please_ submit them together to save review time.

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

Guenter

> ---
>  drivers/watchdog/lpc18xx_wdt.c | 2 +-
>  drivers/watchdog/w83877f_wdt.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/watchdog/lpc18xx_wdt.c b/drivers/watchdog/lpc18xx_wdt.c
> index 78cf11c..60b6d74 100644
> --- a/drivers/watchdog/lpc18xx_wdt.c
> +++ b/drivers/watchdog/lpc18xx_wdt.c
> @@ -292,7 +292,7 @@ static int lpc18xx_wdt_remove(struct platform_device *pdev)
>  	struct lpc18xx_wdt_dev *lpc18xx_wdt = platform_get_drvdata(pdev);
>  
>  	dev_warn(&pdev->dev, "I quit now, hardware will probably reboot!\n");
> -	del_timer(&lpc18xx_wdt->timer);
> +	del_timer_sync(&lpc18xx_wdt->timer);
>  
>  	return 0;
>  }
> diff --git a/drivers/watchdog/w83877f_wdt.c b/drivers/watchdog/w83877f_wdt.c
> index 5772cc5..f265086 100644
> --- a/drivers/watchdog/w83877f_wdt.c
> +++ b/drivers/watchdog/w83877f_wdt.c
> @@ -166,7 +166,7 @@ static void wdt_startup(void)
>  static void wdt_turnoff(void)
>  {
>  	/* Stop the timer */
> -	del_timer(&timer);
> +	del_timer_sync(&timer);
>  
>  	wdt_change(WDT_DISABLE);
>  
> -- 
> 2.6.2
>