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

[Eric Anholt]

Stefan Wahren <stefan.wahren@xxxxxxxx> writes:

> Hi Paul,
>
>> Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx> 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@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)?
>
> 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

We don't share I2C buses with VC4, though, right?  That would just be
totally broken.

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