Re: [PATCH 1/5] clk: renesas: rzv2h: Fix use-after-free in MSTOP refcount handling

[Lad, Prabhakar]

Hi Geert,

On Thu, Dec 19, 2024 at 4:20 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>
> Hi Prabhakar,
>
> On Wed, Dec 18, 2024 at 3:20 PM Prabhakar <prabhakar.csengg@xxxxxxxxx> wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> >
> > Avoid triggering a `refcount_t: addition on 0; use-after-free.` warning
> > when registering a module clock with the same MSTOP configuration. The
> > issue arises when a module clock is registered but not enabled, resulting
> > in a `ref_cnt` of 0. Subsequent calls to `refcount_inc()` on such clocks
> > cause the kernel to warn about use-after-free.
> >
> > [    0.113529] ------------[ cut here ]------------
> > [    0.113537] refcount_t: addition on 0; use-after-free.
> > [    0.113576] WARNING: CPU: 2 PID: 1 at lib/refcount.c:25 refcount_warn_saturate+0x120/0x144
>
> [...]
>
> > Resolve this by checking the `ref_cnt` value before calling
> > `refcount_inc()`. If `ref_cnt` is 0, reset it to 1 using `refcount_set()`.
>
> Thanks for your patch!
>
> > Fixes: 7bd4cb3d6b7c ("clk: renesas: rzv2h: Relocate MSTOP-related macros to the family driver")
>
> The description (from your [PATCH 2/5]?) does not match the commit.
>
Ouch!

> Fixes: 7bd4cb3d6b7c43f0 ("clk: renesas: rzv2h: Add MSTOP support")
>
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
>
> > --- a/drivers/clk/renesas/rzv2h-cpg.c
> > +++ b/drivers/clk/renesas/rzv2h-cpg.c
> > @@ -565,8 +565,12 @@ static struct rzv2h_mstop
> >                         continue;
> >
> >                 if (BUS_MSTOP(clk->mstop->idx, clk->mstop->mask) == mstop_data) {
> > -                       if (rzv2h_mod_clock_is_enabled(&clock->hw))
> > -                               refcount_inc(&clk->mstop->ref_cnt);
> > +                       if (rzv2h_mod_clock_is_enabled(&clock->hw)) {
> > +                               if (refcount_read(&clk->mstop->ref_cnt))
> > +                                       refcount_inc(&clk->mstop->ref_cnt);
> > +                               else
> > +                                       refcount_set(&clk->mstop->ref_cnt, 1);
> > +                       }
> >                         return clk->mstop;
> >                 }
> >         }
>
> This makes me wonder if refcount is the right abstraction?
>
You mean as discussed on irc, refcount per mstop bit instead of groups
is not OK too? Do you have any other better approach in mind?

Cheers,
Prabhakar