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

[Paul Kocialkowski]

Hi,

On Sat, 2018-12-22 at 13:19 +0100, Stefan Wahren wrote:
> Hi Paul,
> 
> > Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >  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)?

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