linux-hardening - Re: [PATCH][next] platform/chrome: Fix -Warray-bounds warnings

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Message-ID: <ZCUzw9epJig2rTIY@google.com>
Date:   Thu, 30 Mar 2023 15:01:23 +0800
From:   Tzung-Bi Shih <tzungbi@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Benson Leung <bleung@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] platform/chrome: Fix -Warray-bounds warnings

On Wed, Mar 29, 2023 at 07:54:02PM -0600, Gustavo A. R. Silva wrote:
> In this case, as only enough space for the op field is allocated,
> we can use an object of type uint32_t instead of a whole
> struct ec_params_vbnvcontext (for which not enough memory is
> allocated).

It doesn't make sense to me.  See comments below.

> Fix the following warning seen under GCC 13:
> drivers/platform/chrome/cros_ec_vbc.c: In function ‘vboot_context_read’:
> drivers/platform/chrome/cros_ec_vbc.c:36:15: warning: array subscript ‘struct ec_params_vbnvcontext[1]’ is partly outside array bounds of ‘unsigned char[36]’ [-Warray-bounds=]
>    36 |         params->op = EC_VBNV_CONTEXT_OP_READ;
>       |               ^~
> In file included from drivers/platform/chrome/cros_ec_vbc.c:12:
> In function ‘kmalloc’,
>     inlined from ‘vboot_context_read’ at drivers/platform/chrome/cros_ec_vbc.c:30:8:
> ./include/linux/slab.h:580:24: note: at offset 20 into object of size 36 allocated by ‘kmalloc_trace’
>   580 |                 return kmalloc_trace(
>       |                        ^~~~~~~~~~~~~~
>   581 |                                 kmalloc_caches[kmalloc_type(flags)][index],
>       |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   582 |                                 flags, size);
>       |                                 ~~~~~~~~~~~~

Please trim the commit message a bit and try to wrap at 75 columns as
[1] suggested.

[1]: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#the-canonical-patch-format

> @@ -20,10 +20,14 @@ static ssize_t vboot_context_read(struct file *filp, struct kobject *kobj,
>  	struct device *dev = kobj_to_dev(kobj);
>  	struct cros_ec_dev *ec = to_cros_ec_dev(dev);
>  	struct cros_ec_device *ecdev = ec->ec_dev;
> -	struct ec_params_vbnvcontext *params;
>  	struct cros_ec_command *msg;
> +	/*
> +	 * This should be a pointer to the same type as op field in
> +	 * struct ec_params_vbnvcontext.
> +	 */
> +	uint32_t *params_op;
>  	int err;
> -	const size_t para_sz = sizeof(params->op);
> +	const size_t para_sz = sizeof(*params_op);
>  	const size_t resp_sz = sizeof(struct ec_response_vbnvcontext);
>  	const size_t payload = max(para_sz, resp_sz);
>  
> @@ -32,8 +36,8 @@ static ssize_t vboot_context_read(struct file *filp, struct kobject *kobj,
>  		return -ENOMEM;
>  
>  	/* NB: we only kmalloc()ated enough space for the op field */
> -	params = (struct ec_params_vbnvcontext *)msg->data;
> -	params->op = EC_VBNV_CONTEXT_OP_READ;
> +	params_op = (uint32_t *)msg->data;
> +	*params_op = EC_VBNV_CONTEXT_OP_READ;

I don't see a good reason to partially allocate memory here.  Perhaps, just
let `para_sz = sizeof(struct ec_params_vbnvcontext)`?  If it also makes
sense to you, please remove the comment "NB: we only..." as well.