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

[Ekansh Gupta <quic_ekangupt@quicinc.com>]

On 6/3/2024 3:32 PM, Dmitry Baryshkov wrote:
> On Mon, Jun 03, 2024 at 11:57:52AM +0530, Ekansh Gupta wrote:
>> On 5/31/2024 5:19 AM, Dmitry Baryshkov wrote:
>>> 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@kernel.org>
>>>> Signed-off-by: Ekansh Gupta <quic_ekangupt@quicinc.com>
>>>> ---
>>>>   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?
>> If the root user sets 0666 for fastrpc nodes, it is expected that this check will get bypassed.
> So, any process will be trusted? This looks so Android-centric. Please come
> with a better way to define 'trusted'.
>
> On a typical UNIX system a used has multiple supplementary GIDs (which
> can be used to allow access to the devices) which have no relationship
> to the process effective GID. On a multi-user machine it might be
> logical that fastrpc nodes have separate group-id and group's read/write
> permissions. But then each of the users has their own unique 'effective'
> GID. Which of those should be using for computing the 'trusted' status?
Thanks for your suggestions, Dmitry. I am considering dropping this patch and system unsignedPD patch
from this series(due to the dependency). I'm redesigning the trusted-process term to make it more generic.
Planning to make it depend on the group IDs and have a check with both primary and supplementary GIDs
of the process. I'll share the design with you along with the changes once it's ready.
>
>>>> +	 * 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?
>> It should be GID, I'll update the comment in next spin.
>>
>>>> +
>>>>   	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
>>>>