linux-kernel - Re: [PATCH] tty: serial: fsl_lpuart: Fix possible integer overflow

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Date:   Tue, 18 Jul 2023 11:33:21 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Sherry Sun <sherry.sun@....com>, gregkh@...uxfoundation.org,
        ilpo.jarvinen@...ux.intel.com, shenwei.wang@....com
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-imx@....com
Subject: Re: [PATCH] tty: serial: fsl_lpuart: Fix possible integer overflow

On 18. 07. 23, 8:56, Sherry Sun wrote:
> This patch addresses the following Coverity report, fix it by casting
> sport->port.frame_time to type u64.
> 
> CID 32305660: Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
> Potentially overflowing expression sport->port.frame_time * 8U with type
> unsigned int (32 bits, unsigned) is evaluated using 32-bit arithmetic,
> and then used in a context that expects an expression of type u64 (64
> bits, unsigned).
> 
> Fixes: cf9aa72d2f91 ("tty: serial: fsl_lpuart: optimize the timer based EOP logic")
> Signed-off-by: Sherry Sun <sherry.sun@....com>
> ---
>   drivers/tty/serial/fsl_lpuart.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> index c1980ea52666..07b3b26732db 100644
> --- a/drivers/tty/serial/fsl_lpuart.c
> +++ b/drivers/tty/serial/fsl_lpuart.c
> @@ -1373,7 +1373,7 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
>   
>   	sport->last_residue = 0;
>   	sport->dma_rx_timeout = max(nsecs_to_jiffies(
> -		sport->port.frame_time * DMA_RX_IDLE_CHARS), 1UL);
> +		(u64)sport->port.frame_time * DMA_RX_IDLE_CHARS), 1UL);

Can you explain how that can overflow? In the worst case (1 start bit, 8 
data bits, 2 stop bits, parity bit, address bit, 50 bauds), frame_time 
would contain:
13*1e9/50 = 260,000,000. (260 ms)

Then the multiplication above is:
260,000,000*8 = 2,080,000,000. (2 seconds)

which is still less than 2^32-1.

thanks,
-- 
js
suse labs