linux-hardening - Re: [PATCH] rtc: msc313: Fix function prototype mismatch in msc313_rtc

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Date:   Thu, 1 Dec 2022 20:46:47 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Daniel Palmer <daniel@...ngy.jp>,
        kernel test robot <lkp@...el.com>,
        Romain Perier <romain.perier@...il.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] rtc: msc313: Fix function prototype mismatch in
 msc313_rtc_probe()

On Sat, Nov 19, 2022 at 09:05:28AM +0100, Christophe JAILLET wrote:
> Le 19/11/2022 à 00:31, Kees Cook a écrit :
> > With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
> > indirect call targets are validated against the expected function
> > pointer prototype to make sure the call target is valid to help mitigate
> > ROP attacks. If they are not identical, there is a failure at run time,
> > which manifests as either a kernel panic or thread getting killed.
> > 
> > msc313_rtc_probe() was passing clk_disable_unprepare() directly, which
> > did not have matching prototypes for devm_add_action_or_reset()'s callback
> > argument. Add a wrapper and remove the cast.
> > 
> > This was found as a result of Clang's new -Wcast-function-type-strict
> > flag, which is more sensitive than the simpler -Wcast-function-type,
> > which only checks for type width mismatches.
> > 
> > Reported-by: kernel test robot <lkp@...el.com>
> > Link: https://lore.kernel.org/lkml/202211041527.HD8TLSE1-lkp@intel.com
> > Cc: Daniel Palmer <daniel@...ngy.jp>
> > Cc: Romain Perier <romain.perier@...il.com>
> > Cc: Alessandro Zummo <a.zummo@...ertech.it>
> > Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
> > Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-rtc@...r.kernel.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >   drivers/rtc/rtc-msc313.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/rtc/rtc-msc313.c b/drivers/rtc/rtc-msc313.c
> > index f3fde013c4b8..36e3e77f303e 100644
> > --- a/drivers/rtc/rtc-msc313.c
> > +++ b/drivers/rtc/rtc-msc313.c
> > @@ -177,6 +177,13 @@ static irqreturn_t msc313_rtc_interrupt(s32 irq, void *dev_id)
> >   	return IRQ_HANDLED;
> >   }
> > +static void msc313_clk_disable_unprepare(void *data)
> > +{
> > +	struct clk *clk = data;
> > +
> > +	clk_disable_unprepare(clk);
> > +}
> > +
> >   static int msc313_rtc_probe(struct platform_device *pdev)
> >   {
> >   	struct device *dev = &pdev->dev;
> > @@ -224,7 +231,7 @@ static int msc313_rtc_probe(struct platform_device *pdev)
> >   		return ret;
> >   	}
> > -	ret = devm_add_action_or_reset(dev, (void (*) (void *))clk_disable_unprepare, clk);
> > +	ret = devm_add_action_or_reset(dev, msc313_clk_disable_unprepare, clk);
> >   	if (ret)
> >   		return ret;
> 
> Hi,
> 
> another way to fix it, is to use devm_clk_get_enabled().
> 
> It removes some LoC instead of introducing some new ones and saves a few
> bytes of memory.

Hrm, I'm not familiar with the clk stuff here -- how do I use it? Should
it just be like this? (The NULL argument is ok?)

diff --git a/drivers/rtc/rtc-msc313.c b/drivers/rtc/rtc-msc313.c
index f3fde013c4b8..8d7737e0e2e0 100644
--- a/drivers/rtc/rtc-msc313.c
+++ b/drivers/rtc/rtc-msc313.c
@@ -212,22 +212,12 @@ static int msc313_rtc_probe(struct platform_device *pdev)
 		return ret;
 	}
 
-	clk = devm_clk_get(dev, NULL);
+	clk = devm_clk_get_enabled(dev, NULL);
 	if (IS_ERR(clk)) {
 		dev_err(dev, "No input reference clock\n");
 		return PTR_ERR(clk);
 	}
 
-	ret = clk_prepare_enable(clk);
-	if (ret) {
-		dev_err(dev, "Failed to enable the reference clock, %d\n", ret);
-		return ret;
-	}
-
-	ret = devm_add_action_or_reset(dev, (void (*) (void *))clk_disable_unprepare, clk);
-	if (ret)
-		return ret;
-
 	rate = clk_get_rate(clk);
 	writew(rate & 0xFFFF, priv->rtc_base + REG_RTC_FREQ_CW_L);
 	writew((rate >> 16) & 0xFFFF, priv->rtc_base + REG_RTC_FREQ_CW_H);

-- 
Kees Cook