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

[Ekansh Gupta]

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@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?

If the root user sets 0666 for fastrpc nodes, it is expected that this check will get bypassed.

> > +	 * 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