Re: [PATCH] clk: meson: pll: Fix by 0 division in __pll_params_to_rate()

Remi Pommarel
2019-12-15T10:52:20Z

On Mon, Dec 09, 2019 at 10:51:13AM +0100, Jerome Brunet wrote:
>
> On Sun 08 Dec 2019 at 22:22, Remi Pommarel <repk@triplefau.lt> wrote:
>
> > Some meson pll registers can be initialized with 0 as N value, introducing
> > the following division by 0 when computing rate :
> >
> > UBSAN: Undefined behaviour in drivers/clk/meson/clk-pll.c:75:9
> > division by zero
> > CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc3-608075-g86c9af8630e1-dirty #400
> > Call trace:
> > dump_backtrace+0x0/0x1c0
> > show_stack+0x14/0x20
> > dump_stack+0xc4/0x100
> > ubsan_epilogue+0x14/0x68
> > __ubsan_handle_divrem_overflow+0x98/0xb8
> > __pll_params_to_rate+0xdc/0x140
> > meson_clk_pll_recalc_rate+0x278/0x3a0
> > __clk_register+0x7c8/0xbb0
> > devm_clk_hw_register+0x54/0xc0
> > meson_eeclkc_probe+0xf4/0x1a0
> > platform_drv_probe+0x54/0xd8
> > really_probe+0x16c/0x438
> > driver_probe_device+0xb0/0xf0
> > device_driver_attach+0x94/0xa0
> > __driver_attach+0x70/0x108
> > bus_for_each_dev+0xd8/0x128
> > driver_attach+0x30/0x40
> > bus_add_driver+0x1b0/0x2d8
> > driver_register+0xbc/0x1d0
> > __platform_driver_register+0x78/0x88
> > axg_driver_init+0x18/0x20
> > do_one_initcall+0xc8/0x24c
> > kernel_init_freeable+0x2b0/0x344
> > kernel_init+0x10/0x128
> > ret_from_fork+0x10/0x18
> >
> > This checks if N is null before doing the division.
>
> Thanks for reporting this
>
> >
> > Fixes: 8289aafa4f36 ("clk: meson: improve pll driver results with
> > frac")
>
> In mainline, the commit above went in with sha1 3c4fe763d64d.
>
> Also, this commit is not really responsible for the problem. Having HW
> initialized with N = 0 would have failed since the beginning, I believe.
>
> In this case the correct fixes would be:
> Fixes: 7a29a869434e ("clk: meson: Add support for Meson clock controller")
>
>
> > Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> > ---
> > drivers/clk/meson/clk-pll.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/clk/meson/clk-pll.c b/drivers/clk/meson/clk-pll.c
> > index ddb1e5634739..6649659f216a 100644
> > --- a/drivers/clk/meson/clk-pll.c
> > +++ b/drivers/clk/meson/clk-pll.c
> > @@ -66,6 +66,10 @@ static unsigned long __pll_params_to_rate(unsigned long parent_rate,
> > (1 << pll->frac.width));
> > }
> >
> > + /* Avoid by zero division */
> > + if (n == 0)
> > + return 0;
>
> This can only really happen after init, in recalc() rate.
>
> I would much prefer if you could check the n value right after it is
> read (meson_parm_read()) in .recalc_rate() and add a comment explaining
> that some HW may have this parameter set 0 on init.
>

Sure will do.

Thanks for reviewing this.

--
Remi

> > +
> > return DIV_ROUND_UP_ULL(rate, n);
> > }
>