linux-kernel - Re: [PATCH] staging: greybus: light: fix a couple double frees

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Date:   Thu, 29 Aug 2019 15:53:20 +0100
From:   Rui Miguel Silva <rmfrfs@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        greybus-dev@...ts.linaro.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] staging: greybus: light: fix a couple double frees

Hi Dan,
On Thu 29 Aug 2019 at 13:28, Dan Carpenter wrote:
> The problem is in gb_lights_request_handler().  If we get a request to
> change the config then we release the light with gb_lights_light_release()
> and re-allocated it.  However, if the allocation fails part way through
> then we call gb_lights_light_release() again.  This can lead to a couple
> different double frees where we haven't cleared out the original values:
>
> 	gb_lights_light_v4l2_unregister(light);
> 	...
> 	kfree(light->channels);
> 	kfree(light->name);
>
> I also made a small change to how we set "light->channels_count = 0;".
> The original code handled this part fine and did not cause a use after
> free but it was sort of complicated to read.
>
> Fixes: 2870b52bae4c ("greybus: lights: add lights implementation")
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
>

Thanks so much for this, I was looking for some time at this and
was half way to a much less elegant fix then yours.

Acked-by: Rui Miguel Silva <rmfrfs@...il.com>

Cheers,
    Rui

> ---
>  drivers/staging/greybus/light.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/staging/greybus/light.c b/drivers/staging/greybus/light.c
> index 010ae1e9c7fb..40680eaf3974 100644
> --- a/drivers/staging/greybus/light.c
> +++ b/drivers/staging/greybus/light.c
> @@ -1098,21 +1098,21 @@ static void gb_lights_channel_release(struct gb_channel *channel)
>  static void gb_lights_light_release(struct gb_light *light)
>  {
>  	int i;
> -	int count;
>
>  	light->ready = false;
>
> -	count = light->channels_count;
> -
>  	if (light->has_flash)
>  		gb_lights_light_v4l2_unregister(light);
> +	light->has_flash = false;
>
> -	for (i = 0; i < count; i++) {
> +	for (i = 0; i < light->channels_count; i++)
>  		gb_lights_channel_release(&light->channels[i]);
> -		light->channels_count--;
> -	}
> +	light->channels_count = 0;
> +
>  	kfree(light->channels);
> +	light->channels = NULL;
>  	kfree(light->name);
> +	light->name = NULL;
>  }
>
>  static void gb_lights_release(struct gb_lights *glights)