Re: [PATCH] i2c: bcm2835: Clear current message and count after a transaction

[Stefan Wahren <stefan.wahren@i2se.com>]

Hi Paul,

> Paul Kocialkowski <paul.kocialkowski@bootlin.com> hat am 24. Dezember 2018 um 10:10 geschrieben:
> 
> 
> Hi,
> 
> On Sat, 2018-12-22 at 13:19 +0100, Stefan Wahren wrote:
> > Hi Paul,
> > 
> > > Paul Kocialkowski <paul.kocialkowski@bootlin.com> hat am 21. Dezember 2018 um 13:11 geschrieben:
> > > 
> > > 
> > > The driver's interrupt handler checks whether a message is currently
> > > being handled with the curr_msg pointer. When it is NULL, the interrupt
> > > is considered to be unexpected. Similarly, the i2c_start_transfer
> > > routine checks for the remaining number of messages to handle in
> > > num_msgs.
> > > 
> > > However, these values are never cleared and always keep the message and
> > > number relevant to the latest transfer (which might be done already and
> > > the underlying message memory might have been freed).
> > > 
> > > When an unexpected interrupt hits with the DONE bit set, the isr will
> > > then try to access the flags field of the curr_msg structure, leading
> > > to a fatal page fault.
> > > 
> > > Fix the issue by systematically clearing curr_msg and num_msgs in the
> > > driver-wide device structure when a transfer is considered complete.
> > > 
> > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > > ---
> > >  drivers/i2c/busses/i2c-bcm2835.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
> > > index 44deae78913e..5486252f5f2f 100644
> > > --- a/drivers/i2c/busses/i2c-bcm2835.c
> > > +++ b/drivers/i2c/busses/i2c-bcm2835.c
> > > @@ -298,6 +298,9 @@ static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
> > >  		return -ETIMEDOUT;
> > >  	}
> > >  
> > > +	i2c_dev->curr_msg = NULL;
> > > +	i2c_dev->num_msgs = 0;
> > 
> > AFAIU this would reduce the chance of a use-after-free dramatically but not completely.
> 
> Do you have a specific case of use-after-free related to these
> variables in mind that this cleanup would not fix (except for the
> timeout case)?

okay i was wrong about the use-after-free. But i'm not sure about this scenario on the I2C bus shared with the VC4:

1. ARM core starts I2C transfer (bcm2835_i2c_start_transfer)
2. VC4 triggers a BCM2835_I2C_S_DONE interrupt
3. ARM core catches this interrupt before the VC4

Stefan

> 
> The msg_buf element (in association with msg_buf_remaining keeping its
> previous value) could also lead to similar problems, so I'm thinking of
> making a bcm2835_i2c_finish_transfer function to group all the cleanups
> and call that right after wait_for_completion_timeout.
> 
> > Btw: Why did you leave of the i2c transfer timeout case?
> 
> I should definitely have included it, actually.
> 
> Cheers,
> 
> Paul
> 
> > Thanks
> > Stefan
> > 
> > > +
> > >  	if (!i2c_dev->msg_err)
> > >  		return num;
> > >  
> > > -- 
> > > 2.20.1
> > > 
> -- 
> Paul Kocialkowski, Bootlin (formerly Free Electrons)
> Embedded Linux and kernel engineering
> https://bootlin.com
>