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

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Thu, Aug 15, 2024 at 07:15:50AM +0200, gregkh@linuxfoundation.org wrote:
> On Thu, Aug 15, 2024 at 02:34:18AM +0000, Selvaraj, Joel (MU-Student) wrote:
> > Hi Srinivas Kandagatla and Ekansh Gupta,
> > 
> > On 6/28/24 06:45, srinivas.kandagatla@linaro.org wrote:
> > > From: Ekansh Gupta <quic_ekangupt@quicinc.com>
> > > 
> > > 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@kernel.org>
> > > Signed-off-by: Ekansh Gupta <quic_ekangupt@quicinc.com>
> > > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > > Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
> > > ---
> > >   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.
> 
> I will be glad to revert it, what was the git id for this in the tree
> now?

Ah, nevermind, I found it, it's bab2f5e8fd5d ("misc: fastrpc: Restrict
untrusted app to attach to privileged PD") and is already in the stable
kernel trees.  Do you want to submit a revert or do you need/want me to
do it?

thanks,

greg k-h