netdev - Re: [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy

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Date: Sat, 23 Sep 2023 20:36:19 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Karsten Keil <isdn@...ux-pingi.de>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] isdn: kcapi: replace deprecated strncpy with strscpy_pad

On Fri, Sep 22, 2023 at 11:49:14AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> `buf` is used in this context as a data buffer with 64 bytes of memory
> to be occupied by capi_manufakturer.
> 
> We see the caller capi20_get_manufacturer() passes data.manufacturer as
> its `buf` argument which is then later passed over to user space. Due to
> this, let's keep the NUL-padding that strncpy provided by using
> strscpy_pad so as to not leak any stack data.
> | 	cdev->errcode = capi20_get_manufacturer(data.contr, data.manufacturer);
> | 	if (cdev->errcode)
> | 		return -EIO;
> |
> | 	if (copy_to_user(argp, data.manufacturer,
> | 			 sizeof(data.manufacturer)))
> | 		return -EFAULT;

Yup, strongly agreed: this needs the padding. I actually wonder if a
follow-up patch might be a good idea here, just for robustness:

 capi_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
         struct capidev *cdev = file->private_data;
-       capi_ioctl_struct data;
+       capi_ioctl_struct data = { };


> 
> Perhaps this would also be a good instance to use `strtomem_pad` for but
> in my testing the compiler was not able to determine the size of `buf`
> -- even with all the hints.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook