linux-kernel - Re: [PATCH][next] usb: gadget: f_fs: Use struct_size() and flex_array

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Date:   Thu, 20 Jan 2022 15:26:53 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] usb: gadget: f_fs: Use struct_size() and
 flex_array_size() helpers

On Thu, Jan 20, 2022 at 04:29:33PM -0600, Gustavo A. R. Silva wrote:
> Make use of the struct_size() and flex_array_size() helpers instead of
> an open-coded version, in order to avoid any potential type mistakes
> or integer overflows that, in the worst scenario, could lead to heap
> overflows.
> 
> Also, address the following sparse warnings:
> drivers/usb/gadget/function/f_fs.c:922:23: warning: using sizeof on a flexible structure
> 
> Link: https://github.com/KSPP/linux/issues/174
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/usb/gadget/function/f_fs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index 25ad1e97a458..7461d27e9604 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -919,12 +919,12 @@ static ssize_t __ffs_epfile_read_data(struct ffs_epfile *epfile,
>  		data_len, ret);
>  
>  	data_len -= ret;
> -	buf = kmalloc(sizeof(*buf) + data_len, GFP_KERNEL);
> +	buf = kmalloc(struct_size(buf, storage, data_len), GFP_KERNEL);
>  	if (!buf)
>  		return -ENOMEM;
>  	buf->length = data_len;
>  	buf->data = buf->storage;
> -	memcpy(buf->storage, data + ret, data_len);
> +	memcpy(buf->storage, data + ret, flex_array_size(buf, storage, data_len));

Looks good to me. This is a place where I think we'll be back later to
fix up the alloc/deserialize-from-memory pattern with a future helper,
but in the meantime, yes, let's get this covered by struct_size().

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

>  
>  	/*
>  	 * At this point read_buffer is NULL or READ_BUFFER_DROP (if
> -- 
> 2.27.0
> 

-- 
Kees Cook