linux-kernel - Re: [PATCH] iio: imu: st_lsm6dsx: fix possible lockup during FIFO read

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Message-ID: <CALKJsrqc__ZeLoZ5V+hBxVMU+Crpv_YG0KM69N1CXuHc_rM-FQ@mail.gmail.com>
Date: Fri, 7 Mar 2025 10:20:34 +0100
From: Silvano Seva <s.seva@...gma.it>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: a.greco@...gma.it, Jonathan Cameron <jic23@...nel.org>, 
	Lars-Peter Clausen <lars@...afoo.de>, 
	"open list:ST LSM6DSx IMU IIO DRIVER" <linux-iio@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iio: imu: st_lsm6dsx: fix possible lockup during FIFO read

On Thu, Mar 6, 2025 at 4:48 PM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> > Prevent st_lsm6dsx_read_fifo and st_lsm6dsx_read_tagged_fifo functions
> > from falling in an infinite loop in case pattern_len is equal to zero and
> > the device FIFO is not empty.
> >
> > Signed-off-by: Silvano Seva <s.seva@...gma.it>
> > ---
> >  drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c
> > index 0a7cd8c1aa33..7f343614f8a5 100644
> > --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c
> > +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c
> > @@ -395,12 +395,17 @@ int st_lsm6dsx_read_fifo(struct st_lsm6dsx_hw *hw)
> >       fifo_len = (le16_to_cpu(fifo_status) & fifo_diff_mask) *
> >                  ST_LSM6DSX_CHAN_SIZE;
> >       fifo_len = (fifo_len / pattern_len) * pattern_len;
> > +     if (!fifo_len)
> > +             return 0;
>
> I do not think this check is necessary since if fifo_len is 0 we will not run
> the for loop, right?

This check is present in the st_lsm6dsx_read_tagged_fifo() function, i
added it here for consistency. I agree with you that is not strictly
necessary.

>
> >
> >       acc_sensor = iio_priv(hw->iio_devs[ST_LSM6DSX_ID_ACC]);
> >       gyro_sensor = iio_priv(hw->iio_devs[ST_LSM6DSX_ID_GYRO]);
> >       if (hw->iio_devs[ST_LSM6DSX_ID_EXT0])
> >               ext_sensor = iio_priv(hw->iio_devs[ST_LSM6DSX_ID_EXT0]);
> >
> > +     if (!pattern_len)
> > +             pattern_len = ST_LSM6DSX_SAMPLE_SIZE;
>
> same here, I do not think pattern_len can be 0 since hw->sip must be greater
> than 0 in order to enable the FIFO. Moreover, this check should be some lines
> above since we have already divided fifo_len by pattern_len here.
>

There is a situation causing the subsequent for loop to never
terminate, hanging the kernel boot process: given a system which
doesn't have an hardware reset line allowing the kernel to
re-initialize the IMU hardware, in case of an hot reboot the driver
probe() function attempts to flush the FIFO, which may contain some
data, while the hw->sip is zero.
The complete execution path is the following:
- call of st_lsm6dsx_probe();
- allocation of the st_lsm6dsx_hw structure via the devm_kzalloc,
zero-initializing the sip field;
- call of st_lsm6dsx_init_device();
- call of st_lsm6dsx_reset_device();
- call of st_lsm6dsx_flush_fifo();
- call of st_lsm6dsx_read_fifo/st_lsm6dsx_read_tagged_fifo via the
fifo_ops function pointer.

An alternative solution to solve this situation is initializing the
hw->sip field to a sane default value in either the probe() or
init_device() function.

> > +
> >       for (read_len = 0; read_len < fifo_len; read_len += pattern_len) {
> >               err = st_lsm6dsx_read_block(hw, ST_LSM6DSX_REG_FIFO_OUTL_ADDR,
> >                                           hw->buff, pattern_len,
> > @@ -623,6 +628,9 @@ int st_lsm6dsx_read_tagged_fifo(struct st_lsm6dsx_hw *hw)
> >       if (!fifo_len)
> >               return 0;
> >
> > +     if (!pattern_len)
> > +             pattern_len = ST_LSM6DSX_TAGGED_SAMPLE_SIZE;
>
> for the reason above, this is not necessary.
>
> Regards,
> Lorenzo
>
> > +
> >       for (read_len = 0; read_len < fifo_len; read_len += pattern_len) {
> >               err = st_lsm6dsx_read_block(hw,
> >                                           ST_LSM6DSX_REG_FIFO_OUT_TAG_ADDR,
> > --
> > 2.48.1
> >