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

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Date:   Sat, 22 Dec 2018 13:19:36 +0100 (CET)
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        Eric Anholt <eric@...olt.net>, linux-i2c@...r.kernel.org
Subject: Re: [PATCH] i2c: bcm2835: Clear current message and count after a
 transaction

Hi Paul,

> Paul Kocialkowski <paul.kocialkowski@...tlin.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@...tlin.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.

Btw: Why did you leave of the i2c transfer timeout case?

Thanks
Stefan

> +
>  	if (!i2c_dev->msg_err)
>  		return num;
>  
> -- 
> 2.20.1
>