nvmem: core: Fix race in nvmem_register()

  nvmem_register() currently registers the device before adding the nvmem
cells, which creates a race window where consumers may find the nvmem
device (and not get PROBE_DEFERred), but then fail to find the cells and
error out.

Move device registration to the end of nvmem_register(), to close the
race.

Observed when the stars line up on Apple Silicon machines with the (not
yet upstream, but trivial) spmi nvmem driver and the macsmc-rtc client:

[    0.487375] macsmc-rtc macsmc-rtc: error -ENOENT: Failed to get rtc_offset NVMEM cell

Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
Signed-off-by: Hector Martin <marcan@marcan.st>
---
 drivers/nvmem/core.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

--
2.35.1
  

On Wed, 14 Dec 2022 at 14:40, Hector Martin <marcan@marcan.st> wrote:
>
> nvmem_register() currently registers the device before adding the nvmem
> cells, which creates a race window where consumers may find the nvmem
> device (and not get PROBE_DEFERred), but then fail to find the cells and
> error out.
>
> Move device registration to the end of nvmem_register(), to close the
> race.
>
> Observed when the stars line up on Apple Silicon machines with the (not
> yet upstream, but trivial) spmi nvmem driver and the macsmc-rtc client:
>
> [    0.487375] macsmc-rtc macsmc-rtc: error -ENOENT: Failed to get rtc_offset NVMEM cell
>
> Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
> Signed-off-by: Hector Martin <marcan@marcan.st>

Reviewed-by: Eric Curtin <ecurtin@redhat.com>

> ---
>  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..d255feca5e17 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -837,20 +837,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>
>         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;
> +                       return ERR_PTR(rval);
>         }
>
>         if (config->compat) {
>                 rval = nvmem_sysfs_setup_compat(nvmem, config);
>                 if (rval)
> -                       goto err_device_del;
> +                       return ERR_PTR(rval);
>         }
>
>         if (config->cells) {
> @@ -867,19 +863,21 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>         if (rval)
>                 goto err_remove_cells;
>
> +       rval = device_register(&nvmem->dev);
> +       if (rval)
> +               goto err_put_device;
> +
>         blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);
>
>         return nvmem;
>
> +err_put_device:
> +       put_device(&nvmem->dev);
>  err_remove_cells:
>         nvmem_device_remove_all_cells(nvmem);
>  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);
>
>         return ERR_PTR(rval);
>  }
> --
> 2.35.1
>
>
  

Thanks Hector for the patch,


On 14/12/2022 14:29, Hector Martin wrote:
> nvmem_register() currently registers the device before adding the nvmem
> cells, which creates a race window where consumers may find the nvmem
> device (and not get PROBE_DEFERred), but then fail to find the cells and
> error out.
> 
> Move device registration to the end of nvmem_register(), to close the
> race.
> 
> Observed when the stars line up on Apple Silicon machines with the (not
> yet upstream, but trivial) spmi nvmem driver and the macsmc-rtc client:
> 
> [    0.487375] macsmc-rtc macsmc-rtc: error -ENOENT: Failed to get rtc_offset NVMEM cell
> 
> Fixes: eace75cfdcf7 ("nvmem: Add a simple NVMEM framework for nvmem providers")
> Signed-off-by: Hector Martin <marcan@marcan.st>

Missing Cc Stable

> ---
>   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..d255feca5e17 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -837,20 +837,16 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
> 
>   	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;
> +			return ERR_PTR(rval);

you can not do this as this will leak ida and nvmem itself.


>   	}
> 
>   	if (config->compat) {
>   		rval = nvmem_sysfs_setup_compat(nvmem, config);
>   		if (rval)
> -			goto err_device_del;
> +			return ERR_PTR(rval);

same here.

>   	}
> 
>   	if (config->cells) {
> @@ -867,19 +863,21 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   	if (rval)
>   		goto err_remove_cells;
> 
> +	rval = device_register(&nvmem->dev);
> +	if (rval)
> +		goto err_put_device;
> +
>   	blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);
> 
>   	return nvmem;
> 
> +err_put_device:
> +	put_device(&nvmem->dev);

this will free nvmem and further down you are going to de-reference the 
freed pointer.

--srini

>   err_remove_cells:
>   	nvmem_device_remove_all_cells(nvmem);
>   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);
> 
>   	return ERR_PTR(rval);
>   }
> --
> 2.35.1
>