linux-kernel - Re: [PATCH] tty: nozomi: avoid sprintf buffer overflow

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Date:   Tue, 29 Nov 2016 08:14:32 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Peter Hurley <peter@...leysoftware.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: nozomi: avoid sprintf buffer overflow

On 11/28/2016, 10:04 PM, Arnd Bergmann wrote:
> Testing with a gcc-7 snapshot produced an internal compiler error
> for this file:
> 
> drivers/tty/nozomi.c: In function 'receive_flow_control':
> drivers/tty/nozomi.c:919:12: internal compiler error: in get_substring_ranges_for_loc, at input.c:1388
>  static int receive_flow_control(struct nozomi *dc)
> 
> I've reported this at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78569
> but also noticed that the code line contains a stack overflow, as it prints
> a string into a slightly shorter fixed-length 'tmp' variable.
> 
> I don't see any point in the temporary variable, we can simply use
> pr_debug() to print the output directly. This change should not change
> any of the output but avoids both the stack overflow and the gcc
> crash.
> 
> The stack overflow will not happen unless a module load parameter is
> also set to enable the debug messages.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/tty/nozomi.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/nozomi.c b/drivers/tty/nozomi.c
> index e2020a691058..17a182b28d52 100644
> --- a/drivers/tty/nozomi.c
> +++ b/drivers/tty/nozomi.c
> @@ -69,13 +69,8 @@
>  #define NOZOMI_DEBUG_LEVEL 0x00
>  
>  #define P_BUF_SIZE 128
> -#define NFO(_err_flag_, args...)				\
> -do {								\
> -	char tmp[P_BUF_SIZE];					\
> -	snprintf(tmp, sizeof(tmp), ##args);			\
> -	printk(_err_flag_ "[%d] %s(): %s\n", __LINE__,		\
> -		__func__, tmp);				\
> -} while (0)
> +#define NFO(_err_flag_, fmt, args...)					\
> +	pr_debug("[%d] %s(): " fmt "\n", __LINE__, __func__,  ##args);

Could you also kill the now unused _err_flag_ parameter (including the
only user of NFO)?

(Or inline pr_debug directly into the only user and kill NFO completely?)

thanks,
-- 
js
suse labs