Re: [PATCH] Sanitize possible_parent_show to Handle Return Value of of_clk_get_parent_name

[Alessandro Carminati]

Hello Stephen,
Thank you for your review and the time you dedicated to it.

Il giorno ven 8 set 2023 alle ore 00:33 Stephen Boyd
<sboyd@xxxxxxxxxx> ha scritto:
>
> Quoting Alessandro Carminati (2023-09-07 07:15:36)
> > this is a gentle ping
> >
>
> I couldn't read your email because it was sent to nobody
> (unlisted-recipients). Can you resend with a proper To: line?
>
> >
> > Il giorno mar 22 ago 2023 alle ore 15:49 Alessandro Carminati
> > <alessandro.carminati@xxxxxxxxx> ha scritto:
> > >
> > > In the possible_parent_show function, ensure proper handling of the return
> > > value from of_clk_get_parent_name to prevent potential issues arising from
> > > a NULL return.
> > > The current implementation invokes seq_puts directly on the result of
> > > of_clk_get_parent_name without verifying the return value, which can lead
> > > to kernel panic if the function returns NULL.
> > >
> > > This patch addresses the concern by introducing a check on the return
> > > value of of_clk_get_parent_name. If the return value is not NULL, the
>
> Use of_clk_get_parent_name() to signify that it is a function.
>
> > > function proceeds to call seq_puts, providing the returned value as
> > > argument.
> > > However, if of_clk_get_parent_name returns NULL, the function provides a
> > > static string as argument, avoiding the panic.
> > >
> > > Reported-by: Philip Daly <pdaly@xxxxxxxxxx>
> > > Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@xxxxxxxxx>
> > > ---
>
> It needs a Fixes tag.
Sure!
I need to be more careful on this.

>
> > >  drivers/clk/clk.c | 11 ++++++-----
> > >  1 file changed, 6 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > index c249f9791ae8..ab999644e185 100644
> > > --- a/drivers/clk/clk.c
> > > +++ b/drivers/clk/clk.c
> > > @@ -3416,6 +3416,7 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
> > >                                  unsigned int i, char terminator)
> > >  {
> > >         struct clk_core *parent;
> > > +       const char *tmp;
> > >
> > >         /*
> > >          * Go through the following options to fetch a parent's name.
> > > @@ -3436,12 +3437,12 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
> > >                 seq_puts(s, core->parents[i].name);
> > >         else if (core->parents[i].fw_name)
> > >                 seq_printf(s, "<%s>(fw)", core->parents[i].fw_name);
> > > -       else if (core->parents[i].index >= 0)
> > > -               seq_puts(s,
> > > -                        of_clk_get_parent_name(core->of_node,
> > > -                                               core->parents[i].index));
> > > -       else
> > > +       else if (core->parents[i].index >= 0) {
> > > +               tmp = of_clk_get_parent_name(core->of_node, core->parents[i].index);
> > > +               seq_puts(s, tmp ? tmp : "(none)");
>
> How about using seq_printf("%s", ...) instead? That should print out
> "(null)" in the case that it is NULL, instead of "(none)" and it is a
> one line change.

I did consider using seq_printf("%s", ...) as an alternative approach to
address the issue initially.
However, after a review of the file's history, I arrived at a different
conclusion.

The commit [1] that introduced this bug was primarily focused on replacing
seq_printf() with seq_putc().
I considered that to maintain code consistency and align with the intentions
of that commit, it may be more appropriate to continue using seq_putc() in
this particular instance.
I agree however with the idea of rolling back that particular change, but
in this case, we perhaps may want to consider also [2], a similar change made
in the same period.
I haven't proceeded with a patch submission for it[2], mainly due to the lack
of evidence of a kernel splash related to it and my uncertainty around the
fact that can exist use cases where the name field in the struct cgroup_subsys
can hit that code set to NULL.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/?id=1ccc0ddf046a0197f2f9acca02a64da10aa6112d
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/?id=85db0023376f529c477c6110043e069ccee16d9c