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Date:   Thu, 14 Dec 2023 08:37:13 -0800
From:   Kees Cook <keescook@chromium.org>
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc:     Benson Leung <bleung@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        chrome-platform@lists.linux.dev, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH][next] platform/chrome: Fix -Warray-bounds warnings
Message-ID: <202312140836.5AB528D54F@keescook>
References: <ZCTrutoN+9TiJM8u@work>
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On Wed, Mar 29, 2023 at 07:54:02PM -0600, Gustavo A. R. Silva wrote:
> GCC-13 (and Clang) does not like having a partially allocated object,
> since it cannot reason about it for bounds checking.
> 
> Notice that the compiler is legitimately complaining about accessing
> an object (params, in this case) for which not enough memory was
> allocated.
> 
> The object is of size 20 bytes:
> 
> struct ec_params_vbnvcontext {
> 	uint32_t                   op;                   /*     0     4 */
> 	uint8_t                    block[16];            /*     4    16 */
> 
> 	/* size: 20, cachelines: 1, members: 2 */
> 	/* last cacheline: 20 bytes */
> };
> 
> but only 16 bytes are allocated:
> 
> sizeof(struct ec_response_vbnvcontext) == 16
> 
> 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).
> 
> 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);
>       |                                 ~~~~~~~~~~~~
> 
> Link: https://github.com/KSPP/linux/issues/278
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

FWIW, I think this is the right change that disrupts the code the least.

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

-Kees

-- 
Kees Cook