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

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Message-ID: <7eygwrefc43aaqeihir3odjhdseeewweqvabvq6jg5bbmnhrkz@p33mhebq5v3w>
Date: Mon, 3 Jun 2024 13:02:31 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Ekansh Gupta <quic_ekangupt@...cinc.com>
Cc: srinivas.kandagatla@...aro.org, linux-arm-msm@...r.kernel.org, 
	gregkh@...uxfoundation.org, quic_bkumar@...cinc.com, linux-kernel@...r.kernel.org, 
	quic_chennak@...cinc.com, stable <stable@...nel.org>
Subject: Re: [PATCH v3 8/9] misc: fastrpc: Restrict untrusted app to spawn
 signed PD

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@...nel.org>
> > > Signed-off-by: Ekansh Gupta <quic_ekangupt@...cinc.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?

> 
> > 
> > > +	 * 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
> > > 

-- 
With best wishes
Dmitry