linux-kernel - Re: [PATCH net] can: can327: fix snprintf() limit in can327_handle

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Message-ID: <6db4d783-6db2-4b86-887c-3c95d6763774@wanadoo.fr>
Date: Thu, 14 Nov 2024 18:34:49 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Dan Carpenter <dan.carpenter@...aro.org>, Max Staudt <max@...as.org>
Cc: Marc Kleine-Budde <mkl@...gutronix.de>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 linux-can@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] can: can327: fix snprintf() limit in
 can327_handle_prompt()

Hi Dan,

On 14/11/2024 at 18:03, Dan Carpenter wrote:
> This code is printing hex values to the &local_txbuf buffer and it's
> using the snprintf() function to try prevent buffer overflows.  The
> problem is that it's not passing the correct limit to the snprintf()
> function so the limit doesn't do anything.  On each iteration we print
> two digits so the remaining size should also decrease by two, but
> instead it passes the sizeof() the entire buffer each time.
> 
> If the frame->len were too long it would result in a buffer overflow.

But, can frame->len be too long? Classical CAN frame maximum length is 8 
bytes. And I do not see a path for a malformed frame to reach this part 
of the driver.

If such a path exists, I think this should be explained. Else, I am just 
not sure if this needs a Fixes: tag.

> I've also changed the function from snprintf() to scnprintf().  The
> difference between the two functions is that snprintf() returns the number
> of bytes which would have been printed if there were space while the
> scnprintf() function returns the number of bytes which are actually
> printed.
> 
> Fixes: 43da2f07622f ("can: can327: CAN/ldisc driver for ELM327 based OBD-II adapters")
> Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
> ---
> ---
>   drivers/net/can/can327.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/can/can327.c b/drivers/net/can/can327.c
> index 24af63961030..5c05ebc72318 100644
> --- a/drivers/net/can/can327.c
> +++ b/drivers/net/can/can327.c
> @@ -623,16 +623,16 @@ static void can327_handle_prompt(struct can327 *elm)
>   			snprintf(local_txbuf, sizeof(local_txbuf), "ATRTR\r");
>   		} else {
>   			/* Send a regular CAN data frame */
> +			int off = 0;
>   			int i;
>   
>   			for (i = 0; i < frame->len; i++) {
> -				snprintf(&local_txbuf[2 * i],
> -					 sizeof(local_txbuf), "%02X",
> -					 frame->data[i]);
> +				off += scnprintf(&local_txbuf[off],
> +						 sizeof(local_txbuf) - off,
> +						 "%02X", frame->data[i]);
>   			}
>   
> -			snprintf(&local_txbuf[2 * i], sizeof(local_txbuf),
> -				 "\r");
> +			scnprintf(&local_txbuf[off], sizeof(local_txbuf) - off, "\r");
>   		}
>   
>   		elm->drop_next_line = 1;

Yours sincerely,
Vincent Mailhol