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Date:   Mon, 11 Oct 2021 11:47:24 +0200
From:   Lorenzo Bianconi <lorenzo@kernel.org>
To:     Lars-Peter Clausen <lars@metafoo.de>
Cc:     Teng Qi <starmiku1207184332@gmail.com>,
        lorenzo.bianconi83@gmail.com, jic23@kernel.org,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        islituo@gmail.com, baijiaju1990@gmail.com,
        TOTE Robot <oslab@tsinghua.edu.cn>
Subject: Re: [PATCH] iio: imu: st_lsm6dsx: Fix an array overflow in
 st_lsm6dsx_set_odr()
Message-ID: <YWQILPRHZw2BsAYu@lore-desk>
References: <20211010091230.895549-1-starmiku1207184332@gmail.com>
 <8efeaffd-ae9a-d0ef-0222-6c94c0e31e7e@metafoo.de>
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> On 10/10/21 11:12 AM, Teng Qi wrote:
> > The length of hw->settings->odr_table is 2 and ref_sensor->id is an enum
> > variable whose value is between 0 and 5.
> > However, the value ST_LSM6DSX_ID_MAX (i.e. 5) is not catched properly in
> >   switch (sensor->id) {
> >=20
> > If ref_sensor->id is ST_LSM6DSX_ID_MAX, an array overflow will ocurrs in
> > function st_lsm6dsx_check_odr():
> >    odr_table =3D &sensor->hw->settings->odr_table[sensor->id];
> >=20
> > and in function st_lsm6dsx_set_odr():
> >    reg =3D &hw->settings->odr_table[ref_sensor->id].reg;
> >=20
> > To fix this possible array overflow, ref_sensor->id should be checked
> > first. If it is greater than or equal to 2, the function
> > st_lsm6dsx_set_odr() returns -EINVAL.
> >=20
> > Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
> > Signed-off-by: Teng Qi <starmiku1207184332@gmail.com>
>=20
> Hi,
>=20
> Thanks for the patch, good catch. But there are a few things to improve
> here, the goal should not be to silence the output of your checker, but to
> write good code.
>=20
> Let's start with ST_LSM6DSX_ID_MAX. As the name suggests this is an entry=
 at
> the end of the enum that is used to get the size of the enum. But its val=
ue
> itself is never assigned to any variable of that type. This is a very com=
mon
> pattern. So strictly speaking there is no bug, since the out-of-range val=
ue
> of ST_LSM6DSX_ID_MAX is never used.
>=20
> The other thing is that we usually don't want to hardcode range checks for
> array sizes with integer literals, but rather use ARRAY_SIZE() instead. T=
his
> makes sure that the code stays correct when the array size changes.
>=20
> If you really wanted to modify the code sot that your code checker does n=
ot
> detect a false positive I'd modify the switch statement to explicitly han=
dle
> ST_LSM6DSX_ID_GYRO and ST_LSM6DSX_ID_ACCEL and the return an error for the
> default case with a comment that default case should never occur.

+1 :)

Regards,
Lorenzo

>=20
> - Lars
>=20
> > ---
> >   drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >=20
> > diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c b/drivers/iio=
/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > index db45f1fc0b81..edf5d33dd256 100644
> > --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > @@ -1308,6 +1308,10 @@ st_lsm6dsx_set_odr(struct st_lsm6dsx_sensor *sen=
sor, u32 req_odr)
> >   		break;
> >   	}
> > +	if (ref_sensor->id >=3D 2) {
> > +		return -EINVAL;
> > +	}
> > +
> >   	if (req_odr > 0) {
> >   		err =3D st_lsm6dsx_check_odr(ref_sensor, req_odr, &val);
> >   		if (err < 0)
>=20
>=20

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