[PATCH] nvmem: fix registration vs use race

[Srinivas Kandagatla]

On 03/01/2023 09:42, Russell King (Oracle) wrote:
> The i.MX6 CPU frequency driver sometimes fails to register at boot time
> due to nvmem_cell_read_u32() sporadically returning -ENOENT.
> 
> This happens because there is a window where __nvmem_device_get() in
> of_nvmem_cell_get() is able to return the nvmem device, but as cells
> have been setup, nvmem_find_cell_entry_by_node() returns NULL.
> 
> The occurs because the nvmem core registration code violates one of the
> fundamental principles of kernel programming: do not publish data
> structures before their setup is complete.
> 
> Fix this by making nvmem core code conform with this principle.
> 
how about a Fixes tag and Cc stable?

> Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
> ---
>   drivers/nvmem/core.c | 18 ++++++++----------
>   1 file changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 321d7d63e068..6b89fb6fa582 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -835,22 +835,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   	nvmem->dev.groups = nvmem_dev_groups;
>   #endif
>   
> -	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> -
> -	rval = device_register(&nvmem->dev);
> -	if (rval)
> -		goto err_put_device;
> -
>   	if (nvmem->nkeepout) {
>   		rval = nvmem_validate_keepouts(nvmem);
>   		if (rval)
> -			goto err_device_del;
> +			goto err_put_device;

AFAIU, as we never did a get_device/kobject_get, calling put_device at 
this point will not invoke a release callback which can potentially leak 
both nvmem and ida.


--srini
>   	}
>   
>   	if (config->compat) {
>   		rval = nvmem_sysfs_setup_compat(nvmem, config);
>   		if (rval)
> -			goto err_device_del;
> +			goto err_put_device;
>   	}
>   
>   	if (config->cells) {
> @@ -867,6 +861,12 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   	if (rval)
>   		goto err_remove_cells;
>   
> +	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> +
> +	rval = device_register(&nvmem->dev);
> +	if (rval)
> +		goto err_remove_cells;
> +
>   	blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);
>   
>   	return nvmem;
> @@ -876,8 +876,6 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   err_teardown_compat:
>   	if (config->compat)
>   		nvmem_sysfs_remove_compat(nvmem, config);
> -err_device_del:
> -	device_del(&nvmem->dev);
>   err_put_device:
>   	put_device(&nvmem->dev);
>