Re: [PATCH v3 8/9] misc: fastrpc: Restrict untrusted app to spawn signed PD

[Dmitry Baryshkov]

On Thu, May 30, 2024 at 03:50:26PM +0530, Ekansh Gupta wrote:
> Some untrusted applications will not have access to open fastrpc
> device nodes and a privileged process can open the device node on
> behalf of the application. Add a check to restrict such untrusted
> applications from offloading to signed PD.
> 
> Fixes: 7f1f481263c3 ("misc: fastrpc: check before loading process to the DSP")
> Cc: stable <stable@xxxxxxxxxx>
> Signed-off-by: Ekansh Gupta <quic_ekangupt@xxxxxxxxxxx>
> ---
>  drivers/misc/fastrpc.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 73fa0e536cf9..32615ccde7ac 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -328,6 +328,7 @@ struct fastrpc_user {
>  	int pd;
>  	bool is_secure_dev;
>  	bool is_unsigned_pd;
> +	bool untrusted_process;
>  	char *servloc_name;
>  	/* Lock for lists */
>  	spinlock_t lock;
> @@ -1249,13 +1250,17 @@ static bool is_session_rejected(struct fastrpc_user *fl, bool unsigned_pd_reques
>  		 * channel is configured as secure and block untrusted apps on channel
>  		 * that does not support unsigned PD offload
>  		 */
> -		if (!fl->cctx->unsigned_support || !unsigned_pd_request) {
> -			dev_err(&fl->cctx->rpdev->dev, "Error: Untrusted application trying to offload to signed PD\n");
> -			return true;
> -		}
> +		if (!fl->cctx->unsigned_support || !unsigned_pd_request)
> +			goto reject_session;
>  	}
> +	/* Check if untrusted process is trying to offload to signed PD */
> +	if (fl->untrusted_process && !unsigned_pd_request)
> +		goto reject_session;
>  
>  	return false;
> +reject_session:
> +	dev_dbg(&fl->cctx->rpdev->dev, "Error: Untrusted application trying to offload to signed PD\n");
> +	return true;
>  }
>  
>  static void fastrpc_mmap_remove_pdr(struct fastrpc_static_pd *spd)
> @@ -1504,12 +1509,20 @@ static int fastrpc_init_create_process(struct fastrpc_user *fl,
>  		goto err;
>  	}
>  
> +	/*
> +	 * Third-party apps don't have permission to open the fastrpc device, so

Permissions depend on the end-user setup. Is it going to break if the
user sets 0666 mode for fastrpc nodes?

> +	 * it is opened on their behalf by a priveleged process. This is detected
> +	 * by comparing current PID with the one stored during device open.
> +	 */
> +	if (current->tgid != fl->tgid)
> +		fl->untrusted_process = true;

If the comment talks about PIDs, when why are you comparing GIDs here?

> +
>  	if (init.attrs & FASTRPC_MODE_UNSIGNED_MODULE)
>  		fl->is_unsigned_pd = true;
>  
>  
>  	if (is_session_rejected(fl, fl->is_unsigned_pd)) {
> -		err = -ECONNREFUSED;
> +		err = -EACCES;
>  		goto err;
>  	}
>  
> -- 
> 2.43.0
> 

-- 
With best wishes
Dmitry