Re: [PATCH 6/6] misc: fastrpc: Restrict untrusted app to attach to privileged PD

[Selvaraj, Joel (MU-Student)]

Hi Srinivas Kandagatla and Ekansh Gupta,

On 6/28/24 06:45, srinivas.kandagatla@xxxxxxxxxx wrote:
> From: Ekansh Gupta <quic_ekangupt@xxxxxxxxxxx>
> 
> Untrusted application with access to only non-secure fastrpc device
> node can attach to root_pd or static PDs if it can make the respective
> init request. This can cause problems as the untrusted application
> can send bad requests to root_pd or static PDs. Add changes to reject
> attach to privileged PDs if the request is being made using non-secure
> fastrpc device node.
> 
> Fixes: 0871561055e6 ("misc: fastrpc: Add support for audiopd")
> Cc: stable <stable@xxxxxxxxxx>
> Signed-off-by: Ekansh Gupta <quic_ekangupt@xxxxxxxxxxx>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx>
> ---
>   drivers/misc/fastrpc.c      | 22 +++++++++++++++++++---
>   include/uapi/misc/fastrpc.h |  3 +++
>   2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 5680856c0fb8..a7a2bcedb37e 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -2087,6 +2087,16 @@ static int fastrpc_req_mem_map(struct fastrpc_user *fl, char __user *argp)
>   	return err;
>   }
>   
> +static int is_attach_rejected(struct fastrpc_user *fl)
> +{
> +	/* Check if the device node is non-secure */
> +	if (!fl->is_secure_dev) {
> +		dev_dbg(&fl->cctx->rpdev->dev, "untrusted app trying to attach to privileged DSP PD\n");
> +		return -EACCES;
> +	}
> +	return 0;
> +}

This broke userspace for us. Sensors stopped working in SDM845 and other 
qcom SoC devices running postmarketOS. Trying to communicate with the 
fastrpc device just ends up with a permission denied error. This was 
previously working. I am not sure if this is intended. Here are my two 
observations:

1. if change the if condition to

`if (!fl->is_secure_dev && fl->cctx->secure)`

similar to how it's done in fastrpc's `is_session_rejected()` function, 
then it works. But I am not sure if this is an valid fix. But currently, 
fastrpc will simply deny access to all fastrpc device that contains the 
`qcom,non-secure-domain` dt property. Is that the intended change? 
Because I see a lot of adsp, cdsp and sdsp fastrpc nodes have that dt 
property.

2. In the `fastrpc_rpmsg_probe()` function, it is commented that,

"Unsigned PD offloading is only supported on CDSP"

Does this mean adsp and sdsp shouldn't have the `qcom,non-secure-domain` 
dt property? In fact, it was reported that removing this dt property and 
using the `/dev/fastrpc-sdsp-secure` node instead works fine too. Is 
this the correct way to fix it?

I don't know much about fastrpc, just reporting the issue and guessing 
here. It would be really if this can be fixed before the stable release.

Thank you,
Joel Selvaraj