lkml.org 
[lkml]   [2018]   [Dec]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] i2c: bcm2835: Clear current message and count after a transaction
Date
Stefan Wahren <stefan.wahren@i2se.com> writes:

> 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

We don't share I2C buses with VC4, though, right? That would just be
totally broken.
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2018-12-27 19:16    [W:0.058 / U:13.924 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site