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

Kees Cook
2023-12-14T16:37:20Z

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