Re: [PATCH] watchdog: aspeed: fix integer overflow in set_timeout handler

From: Tao Ren <rentao.bupt@gmail.com>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>,
	Joel Stanley <joel@jms.id.au>, Andrew Jeffery <andrew@aj.id.au>,
	linux-watchdog@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-aspeed@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	openbmc@lists.ozlabs.org, Tao Ren <taoren@fb.com>,
	Amithash Prasad <amithash@fb.com>
Subject: Re: [PATCH] watchdog: aspeed: fix integer overflow in set_timeout handler
Date: Thu, 15 Apr 2021 18:04:45 -0700	[thread overview]
Message-ID: <20210416010444.GA17388@taoren-ubuntu-R90MNF91> (raw)
In-Reply-To: <469ac948-d65b-471f-102f-726466c19c5c@roeck-us.net>

On Thu, Apr 15, 2021 at 05:50:32PM -0700, Guenter Roeck wrote:
> On 4/15/21 5:12 PM, rentao.bupt@gmail.com wrote:
> > From: Tao Ren <rentao.bupt@gmail.com>
> > 
> > Fix the time comparison (timeout vs. max_hw_heartbeat_ms) in set_timeout
> > handler to avoid potential integer overflow when the supplied timeout is
> > greater than aspeed's maximum allowed timeout (4294 seconds).
> > 
> > Fixes: efa859f7d786 ("watchdog: Add Aspeed watchdog driver")
> > Reported-by: Amithash Prasad <amithash@fb.com>
> > Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
> > ---
> >  drivers/watchdog/aspeed_wdt.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/watchdog/aspeed_wdt.c b/drivers/watchdog/aspeed_wdt.c
> > index 7e00960651fa..9f77272dc906 100644
> > --- a/drivers/watchdog/aspeed_wdt.c
> > +++ b/drivers/watchdog/aspeed_wdt.c
> > @@ -145,9 +145,8 @@ static int aspeed_wdt_set_timeout(struct watchdog_device *wdd,
> >  	struct aspeed_wdt *wdt = to_aspeed_wdt(wdd);
> >  	u32 actual;
> >  
> > -	wdd->timeout = timeout;
> > -
> > -	actual = min(timeout, wdd->max_hw_heartbeat_ms * 1000);
> > +	actual = min(timeout, wdd->max_hw_heartbeat_ms / 1000);
> > +	wdd->timeout = actual;
> >  
> >  	writel(actual * WDT_RATE_1MHZ, wdt->base + WDT_RELOAD_VALUE);
> >  	writel(WDT_RESTART_MAGIC, wdt->base + WDT_RESTART);
> > 
> 
> If the provided timeout is larger than the supported hardware timeout,
> the watchdog core will ping the hardware on behalf of userspace.
> The above code would defeat that mechanism for no good reason.
> 
> NACK.
> 
> Guenter

Thanks Guenter for Joel for the quick review!

The integer overflow happens at (actual * WDT_RATE_1MHZ). For example,
if a user tries to set timeout to 4295 seconds, then the hardware would
be programmed to timeout after about 32 milliseconds. I would say this
behavior is not expected?

Cheers,

Tao

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