linux-kernel - Re: [PATCH] USB: wusbcore: crypto: Remove VLA usage

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Date:   Fri, 16 Mar 2018 08:19:31 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: wusbcore: crypto: Remove VLA usage


I just discovered an issue with this patch. Please, drop it. I'll send 
v2 shortly.

Thanks
--
Gustavo

On 03/16/2018 08:01 AM, Gustavo A. R. Silva wrote:
> In preparation to enabling -Wvla, remove VLA and replace it
> with dynamic memory allocation instead.
> 
> The use of stack Variable Length Arrays needs to be avoided, as they
> can be a vector for stack exhaustion, which can be both a runtime bug
> or a security flaw. Also, in general, as code evolves it is easy to
> lose track of how big a VLA can get. Thus, we can end up having runtime
> failures that are hard to debug.
> 
> Also, fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621
> 
> Notice that in this particular case, an alternative to kzalloc is kcalloc,
> in which case the code would look as follows instead:
> 
> iv = kcalloc(crypto_skcipher_ivsize(tfm_cbc), sizeof(*iv), GFP_KERNEL);
> 
> but if the data type of _iv_ never changes, or the type size is always one
> byte, kzalloc is good enough.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>   drivers/usb/wusbcore/crypto.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/wusbcore/crypto.c b/drivers/usb/wusbcore/crypto.c
> index 4c00be2d..3511473 100644
> --- a/drivers/usb/wusbcore/crypto.c
> +++ b/drivers/usb/wusbcore/crypto.c
> @@ -202,7 +202,7 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
>   	struct scatterlist sg[4], sg_dst;
>   	void *dst_buf;
>   	size_t dst_size;
> -	u8 iv[crypto_skcipher_ivsize(tfm_cbc)];
> +	u8 *iv;
>   	size_t zero_padding;
>   
>   	/*
> @@ -222,9 +222,11 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
>   		zero_padding;
>   	dst_buf = kzalloc(dst_size, GFP_KERNEL);
>   	if (!dst_buf)
> -		goto error_dst_buf;
> +		goto error_alloc;
>   
> -	memset(iv, 0, sizeof(iv));
> +	iv = kzalloc(crypto_skcipher_ivsize(tfm_cbc), GFP_KERNEL);
> +	if (!iv)
> +		goto error_alloc;
>   
>   	/* Setup B0 */
>   	scratch->b0.flags = 0x59;	/* Format B0 */
> @@ -276,8 +278,9 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
>   	bytewise_xor(mic, &scratch->ax, iv, 8);
>   	result = 8;
>   error_cbc_crypt:
> +	kfree(iv);
>   	kfree(dst_buf);
> -error_dst_buf:
> +error_alloc:
>   	return result;
>   }
>   
>