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From:   Eric Anholt <eric@anholt.net>
To:     Stefan Wahren <stefan.wahren@i2se.com>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc:     Florian Fainelli <f.fainelli@gmail.com>,
        Ray Jui <rjui@broadcom.com>,
        Scott Branden <sbranden@broadcom.com>,
        linux-kernel@vger.kernel.org,
        bcm-kernel-feedback-list@broadcom.com,
        linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-i2c@vger.kernel.org
Subject: Re: [PATCH] i2c: bcm2835: Clear current message and count after a transaction
In-Reply-To: <1232214204.268730.1545919526431@email.ionos.de>
References: <20181221121135.4847-1-paul.kocialkowski@bootlin.com> <1941132339.131316.1545481176339@email.ionos.de> <baa17fb765bf8b40fe6e91b4884b83af6fcf7641.camel@bootlin.com> <1232214204.268730.1545919526431@email.ionos.de>
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Date:   Thu, 27 Dec 2018 10:15:44 -0800
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Stefan Wahren <stefan.wahren@i2se.com> writes:

> Hi Paul,
>
>> Paul Kocialkowski <paul.kocialkowski@bootlin.com> hat am 24. Dezember 20=
18 um 10:10 geschrieben:
>>=20
>>=20
>> Hi,
>>=20
>> On Sat, 2018-12-22 at 13:19 +0100, Stefan Wahren wrote:
>> > Hi Paul,
>> >=20
>> > > Paul Kocialkowski <paul.kocialkowski@bootlin.com> hat am 21. Dezembe=
r 2018 um 13:11 geschrieben:
>> > >=20
>> > >=20
>> > > The driver's interrupt handler checks whether a message is currently
>> > > being handled with the curr_msg pointer. When it is NULL, the interr=
upt
>> > > is considered to be unexpected. Similarly, the i2c_start_transfer
>> > > routine checks for the remaining number of messages to handle in
>> > > num_msgs.
>> > >=20
>> > > 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).
>> > >=20
>> > > 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.
>> > >=20
>> > > Fix the issue by systematically clearing curr_msg and num_msgs in the
>> > > driver-wide device structure when a transfer is considered complete.
>> > >=20
>> > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
>> > > ---
>> > >  drivers/i2c/busses/i2c-bcm2835.c | 3 +++
>> > >  1 file changed, 3 insertions(+)
>> > >=20
>> > > diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i=
2c-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;
>> > >  	}
>> > >=20=20
>> > > +	i2c_dev->curr_msg =3D NULL;
>> > > +	i2c_dev->num_msgs =3D 0;
>> >=20
>> > AFAIU this would reduce the chance of a use-after-free dramatically bu=
t not completely.
>>=20
>> 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 sc=
enario 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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