linux-hardening - Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

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Date:   Tue, 13 Jun 2023 12:30:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Azeem Shaikh <azeemshaikh38@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-hardening@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Albert Briscoe <albertsbriscoe@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>
Subject: Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with
 strscpy

On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit.
> This is both inefficient and can lead to linear read
> overflows if a source string is not NUL-terminated [1].
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
> 
> Direct replacement is safe here since return value of -E2BIG
> is used to check for truncation instead of sizeof(dest).

Perhaps say "PAGE_SIZE" here instead of sizeof(dest)?

> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> 
> Signed-off-by: Azeem Shaikh <azeemshaikh38@...il.com>
> ---
>  drivers/usb/gadget/function/f_printer.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
> index 28db3e336e7d..10dc7c55c25f 100644
> --- a/drivers/usb/gadget/function/f_printer.c
> +++ b/drivers/usb/gadget/function/f_printer.c
> @@ -1211,8 +1211,8 @@ static ssize_t f_printer_opts_pnp_string_show(struct config_item *item,
>  	if (!opts->pnp_string)
>  		goto unlock;
>  
> -	result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> -	if (result >= PAGE_SIZE) {
> +	result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> +	if (result == -E2BIG) {

I think "< 1" might be a better test here.

>  		result = PAGE_SIZE;
>  	} else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {
>  		page[result++] = '\n';
> -- 
> 2.41.0.162.gfafddb0af9-goog
> 
> 

-Kees

-- 
Kees Cook