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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Thu, Mar 6, 2025 at 4:48 PM Lorenzo Bianconi <lorenzo@kernel.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@4sigma.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.

ack, I can see the issue now, thx for pointing this out. I should we should set
pattern_len to ST_LSM6DSX_SAMPLE_SIZE or ST_LSM6DSX_TAGGED_SAMPLE_SIZE if it is
0. Can you please move the check before updating fifo_len in
st_lsm6dsx_read_fifo()?

Regards,
Lorenzo

> 
> 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
> > >

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