linux-kernel - Re: [PATCH 1/8] bus: mhi: core: Validate channel ID when processing command completions

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Message-ID: <20210624155602.GA28535@thinkpad>
Date:   Thu, 24 Jun 2021 21:26:02 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     hemantk@...eaurora.org, bbhatt@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        loic.poulain@...aro.org, stable@...r.kernel.org,
        Jeffrey Hugo <quic_jhugo@...cinc.com>
Subject: Re: [PATCH 1/8] bus: mhi: core: Validate channel ID when processing
 command completions

On Thu, Jun 24, 2021 at 05:27:17PM +0200, Greg KH wrote:
> On Thu, Jun 24, 2021 at 08:17:52PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Jun 24, 2021 at 04:39:51PM +0200, Greg KH wrote:
> > > On Thu, Jun 24, 2021 at 08:02:48PM +0530, Manivannan Sadhasivam wrote:
> > > > On Thu, Jun 24, 2021 at 03:50:45PM +0200, Greg KH wrote:
> > > > > On Mon, Jun 21, 2021 at 09:46:09PM +0530, Manivannan Sadhasivam wrote:
> > > > > > From: Bhaumik Bhatt <bbhatt@...eaurora.org>
> > > > > > 
> > > > > > MHI reads the channel ID from the event ring element sent by the
> > > > > > device which can be any value between 0 and 255. In order to
> > > > > > prevent any out of bound accesses, add a check against the maximum
> > > > > > number of channels supported by the controller and those channels
> > > > > > not configured yet so as to skip processing of that event ring
> > > > > > element.
> > > > > > 
> > > > > > Cc: stable@...r.kernel.org
> > > > > > Fixes: 1d3173a3bae7 ("bus: mhi: core: Add support for processing events from client device")
> > > > > > Signed-off-by: Bhaumik Bhatt <bbhatt@...eaurora.org>
> > > > > > Reviewed-by: Hemant Kumar <hemantk@...eaurora.org>
> > > > > > Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > > > Reviewed-by: Jeffrey Hugo <quic_jhugo@...cinc.com>
> > > > > > Link: https://lore.kernel.org/r/1619481538-4435-1-git-send-email-bbhatt@codeaurora.org
> > > > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > > > ---
> > > > > >  drivers/bus/mhi/core/main.c | 15 ++++++++++-----
> > > > > >  1 file changed, 10 insertions(+), 5 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/bus/mhi/core/main.c b/drivers/bus/mhi/core/main.c
> > > > > > index 22acde118bc3..ed07421c4870 100644
> > > > > > --- a/drivers/bus/mhi/core/main.c
> > > > > > +++ b/drivers/bus/mhi/core/main.c
> > > > > > @@ -773,11 +773,16 @@ static void mhi_process_cmd_completion(struct mhi_controller *mhi_cntrl,
> > > > > >  	cmd_pkt = mhi_to_virtual(mhi_ring, ptr);
> > > > > >  
> > > > > >  	chan = MHI_TRE_GET_CMD_CHID(cmd_pkt);
> > > > > > -	mhi_chan = &mhi_cntrl->mhi_chan[chan];
> > > > > > -	write_lock_bh(&mhi_chan->lock);
> > > > > > -	mhi_chan->ccs = MHI_TRE_GET_EV_CODE(tre);
> > > > > > -	complete(&mhi_chan->completion);
> > > > > > -	write_unlock_bh(&mhi_chan->lock);
> > > > > > +	WARN_ON(chan >= mhi_cntrl->max_chan);
> > > > > 
> > > > > What can ever trigger this WARN_ON()?  Do you mean to reboot a machine
> > > > > if panic-on-warn is set?
> > > > > 
> > > > > If this can actually happen, then check for it and recover properly,
> > > > > don't just blindly warn and then keep on going.
> > > > > 
> > > > 
> > > > We can't do much here other than warning the user and dropping the
> > > > command.
> > > 
> > > But you didn't warn anyone.  Well, you rebooted the machine, is that ok?
> > > If this can be triggered by a user, this should never happen.
> > > 
> > > Do not use WARN_ON() ever please.
> > > 
> > > > There is no recovery possible because, the device has sent the command
> > > > completion event on a wrong channel. It can't happen usually unless a
> > > > malcious device sits on the other end.
> > > 
> > > Then just eat the message and move on, please do not crash the box.
> > > 
> > 
> > Okay. We'll spit an error message and drop the event.
> 
> If this can be triggered by a user, don't provide a way to DoS the
> kernel error log.
> 

The term "user" is a bit vague here. Only a malcious device sits on the PCIe
bus that claims a defined VID/PID can trigger this error. And we do need to tell
the user of the host machine that the device tried to do something wrong.

So I guess the error log is fine here?

Thanks,
Mani

> thanks,
> 
> greg k-h