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

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Message-ID: <CAMuHMdVz95gXsYpF57sDJ4Y_by0chEuzgN-Bz-KZpzycZMrtGQ@mail.gmail.com>
Date: Thu, 19 Dec 2024 17:20:08 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Prabhakar <prabhakar.csengg@...il.com>
Cc: Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	linux-renesas-soc@...r.kernel.org, linux-clk@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Biju Das <biju.das.jz@...renesas.com>, 
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH 1/5] clk: renesas: rzv2h: Fix use-after-free in MSTOP
 refcount handling

Hi Prabhakar,

On Wed, Dec 18, 2024 at 3:20 PM Prabhakar <prabhakar.csengg@...il.com> wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
>
> 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.

Fixes: 7bd4cb3d6b7c43f0 ("clk: renesas: rzv2h: Add MSTOP support")

> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>

> --- 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?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds