linux-kernel - Re: [PATCH 20/20] s390/watchdog: Manual replacement of the deprecated strlcpy() with return values

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Message-ID: <98317dfa-d884-d4ae-9449-d84b902c0e5c@roeck-us.net>
Date:   Mon, 22 Feb 2021 07:55:49 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Romain Perier <romain.perier@...il.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 20/20] s390/watchdog: Manual replacement of the deprecated
 strlcpy() with return values

On 2/22/21 7:12 AM, Romain Perier wrote:
> The strlcpy() reads the entire source buffer first, it is dangerous if
> the source buffer lenght is unbounded or possibility non NULL-terminated.

length

> It can lead to linear read overflows, crashes, etc...
> 

That won't happen here since both buffers have the same length
and the source is known to be NULL_terminated.

So this is another misleading patch. If strlcpy() is deemed so dangerous
nowadays, replace it with memcpy(). That is expensive too, but at least
it won't create the impression that a non-existing error would ever occur
On top of that, the code is handling all MAX_CMDLEN bytes twice anyway
later on, so we can for sure afford the extra cost of the memcpy().
Pus, we'll have the added benefit that the unused part of the buffer
will be cleared and no longer contain random data.

Guenter

> As recommended in the deprecated interfaces [1], it should be replaced
> by strscpy.
> 
> This commit replaces all calls to strlcpy that handle the return values
> by the corresponding strscpy calls with new handling of the return
> values (as it is quite different between the two functions).
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> 
> Signed-off-by: Romain Perier <romain.perier@...il.com>
> ---
>  drivers/watchdog/diag288_wdt.c |   12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/watchdog/diag288_wdt.c b/drivers/watchdog/diag288_wdt.c
> index aafc8d98bf9f..5703f35dd0b7 100644
> --- a/drivers/watchdog/diag288_wdt.c
> +++ b/drivers/watchdog/diag288_wdt.c
> @@ -111,7 +111,7 @@ static unsigned long wdt_status;
>  static int wdt_start(struct watchdog_device *dev)
>  {
>  	char *ebc_cmd;
> -	size_t len;
> +	ssize_t len;
>  	int ret;
>  	unsigned int func;
>  
> @@ -126,7 +126,9 @@ static int wdt_start(struct watchdog_device *dev)
>  			clear_bit(DIAG_WDOG_BUSY, &wdt_status);
>  			return -ENOMEM;
>  		}
> -		len = strlcpy(ebc_cmd, wdt_cmd, MAX_CMDLEN);
> +		len = strscpy(ebc_cmd, wdt_cmd, MAX_CMDLEN);
> +		if (len == -E2BIG)
> +			return -E2BIG;
>  		ASCEBC(ebc_cmd, MAX_CMDLEN);
>  		EBC_TOUPPER(ebc_cmd, MAX_CMDLEN);
>  
> @@ -163,7 +165,7 @@ static int wdt_stop(struct watchdog_device *dev)
>  static int wdt_ping(struct watchdog_device *dev)
>  {
>  	char *ebc_cmd;
> -	size_t len;
> +	ssize_t len;
>  	int ret;
>  	unsigned int func;
>  
> @@ -173,7 +175,9 @@ static int wdt_ping(struct watchdog_device *dev)
>  		ebc_cmd = kmalloc(MAX_CMDLEN, GFP_KERNEL);
>  		if (!ebc_cmd)
>  			return -ENOMEM;
> -		len = strlcpy(ebc_cmd, wdt_cmd, MAX_CMDLEN);
> +		len = strscpy(ebc_cmd, wdt_cmd, MAX_CMDLEN);
> +		if (len == -E2BIG)
> +			return -E2BIG;
>  		ASCEBC(ebc_cmd, MAX_CMDLEN);
>  		EBC_TOUPPER(ebc_cmd, MAX_CMDLEN);
>  
>