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

Kees Cook
2023-12-14T16:35:24Z

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>

This patch seems to have gotten lost? Looking at the conversation, I
think it should land as-is rather than changing the allocation size.

I can pick this up via my tree if that helps...

-Kees

> ---
> drivers/platform/chrome/cros_ec_vbc.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/platform/chrome/cros_ec_vbc.c b/drivers/platform/chrome/cros_ec_vbc.c
> index c859c862d7ac..b5a584f5469a 100644
> --- a/drivers/platform/chrome/cros_ec_vbc.c
> +++ b/drivers/platform/chrome/cros_ec_vbc.c
> @@ -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;
>
> msg->version = EC_VER_VBNV_CONTEXT;
> msg->command = EC_CMD_VBNV_CONTEXT;
> --
> 2.34.1
>

--
Kees Cook