linux-kernel - Re: [PATCH 1/2] tty: serial: fsl_lpuart: fix the potential bug of division or modulo by zero

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Date:   Mon, 26 Apr 2021 14:23:11 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Sherry Sun <sherry.sun@....com>
Cc:     "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH 1/2] tty: serial: fsl_lpuart: fix the potential bug of
 division or modulo by zero

On Mon, Apr 26, 2021 at 11:51:39AM +0000, Sherry Sun wrote:
> Hi Greg,
> 
> > > > >  drivers/tty/serial/fsl_lpuart.c | 3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > >
> > > > > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > > > > b/drivers/tty/serial/fsl_lpuart.c index 794035041744..777d54b593f8
> > > > > 100644
> > > > > --- a/drivers/tty/serial/fsl_lpuart.c
> > > > > +++ b/drivers/tty/serial/fsl_lpuart.c
> > > > > @@ -2414,6 +2414,9 @@ lpuart32_console_get_options(struct
> > > > > lpuart_port *sport, int *baud,
> > > > >
> > > > >  	bd = lpuart32_read(&sport->port, UARTBAUD);
> > > > >  	bd &= UARTBAUD_SBR_MASK;
> > > > > +	if (!bd)
> > > > > +		return;
> > > >
> > > > How can this ever happen?
> > > >
> > > > Not to say this is a bad check, but it feels like this can't really
> > > > happen in real life, what code patch could create this result?
> > > >
> > > > And have you tested this on real hardware?
> > > >
> > >
> > > Thanks for the reviewing, yes, I have tested the patchset on the real
> > hardware.
> > >
> > > Seems the coverity check is static scan, so cannot judge if UARTBAUD
> > Register will be zero.
> > > I just found below statement in the uart reference manual: "When SBR is 1
> > - 8191, the baud rate equals "baud clock / ((OSR+1) × SBR)"."
> > > Since I am not familiar with uart, do you mean that the value of UARTBAUD
> > Register will never be zero, so this case will not happen in real word?
> > 
> > Given that this never has happened with hardware for such an old device,
> > perhaps it is impossible.  But it would be good to check.
> > 
> > > If yes, I will drop this patch.
> > 
> > Handling "bad data" from hardware is never a bad idea, so I don't
> > necessarily want to drop this patch, I just want to try to figure out if this is a
> > "incase the hardware is broken/malicious" type of change, vs.
> > a "this bug we are seeing in real hardware" type of change.
> > 
> 
> Yes, you are right, the probability of hardware happen in this case is really low. But we cannot guarantee that it will never happen.
> So will this check here be accepted? Thanks!

Please resubmit it with a better changelog description summarizing the
discussion here to make it more obvious why this change is needed.

thanks,

greg k-h