Re: [PATCH] drivers/i2c-aspeed: avoid invalid memory reference after timeout

[Joel Stanley <joel@jms.id.au>]

On Fri, 14 Jan 2022 at 14:01, Heyi Guo <guoheyi@linux.alibaba.com> wrote:
>
> Hi Joel,
>
>
> 在 2022/1/11 下午6:51, Joel Stanley 写道:
> > On Tue, 11 Jan 2022 at 07:52, Heyi Guo <guoheyi@linux.alibaba.com> wrote:
> >> Hi all,
> >>
> >> Any comments?
> >>
> >> Thanks,
> >>
> >> Heyi
> >>
> >> 在 2022/1/9 下午9:26, Heyi Guo 写道:
> >>> The memory will be freed by the caller if transfer timeout occurs,
> >>> then it would trigger kernel panic if the peer device responds with
> >>> something after timeout and triggers the interrupt handler of aspeed
> >>> i2c driver.
> >>>
> >>> Set the msgs pointer to NULL to avoid invalid memory reference after
> >>> timeout to fix this potential kernel panic.
> > Thanks for the patch. How did you discover this issue? Do you have a
> > test I can run to reproduce the crash?
>
> We are using one i2c channel to communicate with another MCU by
> implementing user space SSIF protocol, and the MCU may not respond in
> time if it is busy. If it responds after timeout occurs, it will trigger
> below kernel panic:
>

Thanks for the details. It looks like you've done some testing of
this, which is good.

> After applying this patch, we'll get below warning instead:
>
> "bus in unknown state. irq_status: 0x%x\n"

Given we get to here in the irq handler, we've done these two tests:

 - aspeed_i2c_is_irq_error()
 - the state is not INACTIVE or PENDING

but there's no buffer ready for us to use. So what has triggered the
IRQ in this case? Do you have a record of the irq status bits?

I am wondering if the driver should know that the transaction has
timed out, instead of detecting this unknown state.


> > Can you provide a Fixes tag?
>
> I think the bug was introduced by the first commit of this file :(
>
> f327c686d3ba44eda79a2d9e02a6a242e0b75787
>
>
> >
> > Do other i2c master drivers do this? I took a quick look at the meson
> > driver and it doesn't appear to clear it's pointer to msgs.
>
> It is hard to say. It seems other drivers have some recover scheme like
> aborting the transfer, or loop each messages in process context and
> don't do much in IRQ handler, which may disable interrupts or not retain
> the buffer pointer before returning timeout.

I think your change is okay to go in as it fixes the crash, but first
I want to work out if there's some missing handling of a timeout
condition that we should add as well.


>
> Thanks,
>
> Heyi
>
>
> >
> >>> Signed-off-by: Heyi Guo <guoheyi@linux.alibaba.com>
> >>>
> >>> -------
> >>>
> >>> Cc: Brendan Higgins <brendanhiggins@google.com>
> >>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> >>> Cc: Joel Stanley <joel@jms.id.au>
> >>> Cc: Andrew Jeffery <andrew@aj.id.au>
> >>> Cc: Philipp Zabel <p.zabel@pengutronix.de>
> >>> Cc: linux-i2c@vger.kernel.org
> >>> Cc: openbmc@lists.ozlabs.org
> >>> Cc: linux-arm-kernel@lists.infradead.org
> >>> Cc: linux-aspeed@lists.ozlabs.org
> >>> ---
> >>>    drivers/i2c/busses/i2c-aspeed.c | 5 +++++
> >>>    1 file changed, 5 insertions(+)
> >>>
> >>> diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
> >>> index 67e8b97c0c950..3ab0396168680 100644
> >>> --- a/drivers/i2c/busses/i2c-aspeed.c
> >>> +++ b/drivers/i2c/busses/i2c-aspeed.c
> >>> @@ -708,6 +708,11 @@ static int aspeed_i2c_master_xfer(struct i2c_adapter *adap,
> >>>                spin_lock_irqsave(&bus->lock, flags);
> >>>                if (bus->master_state == ASPEED_I2C_MASTER_PENDING)
> >>>                        bus->master_state = ASPEED_I2C_MASTER_INACTIVE;
> >>> +             /*
> >>> +              * All the buffers may be freed after returning to caller, so
> >>> +              * set msgs to NULL to avoid memory reference after freeing.
> >>> +              */
> >>> +             bus->msgs = NULL;
> >>>                spin_unlock_irqrestore(&bus->lock, flags);
> >>>
> >>>                return -ETIMEDOUT;