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. Refactor to use devm_clk_get_enabled() instead.

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@intel.com>
Link: https://lore.kernel.org/lkml/202211041527.HD8TLSE1-lkp@intel.com
Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Daniel Palmer <daniel@thingy.jp>
Cc: Romain Perier <romain.perier@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rtc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v1: https://lore.kernel.org/lkml/20221118233101.never.215-kees@kernel.org/
v2: use devm_clk_get_enabled() (christophe)
---
 drivers/rtc/rtc-msc313.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

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);
-- 
2.34.1

On Fri, 2 Dec 2022 10:45:30 -0800, Kees Cook wrote:
> 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. Refactor to use devm_clk_get_enabled() instead.
> 
> [...]

Applied, thanks!

[1/1] rtc: msc313: Fix function prototype mismatch in msc313_rtc_probe()
      commit: 21b8a1dd56a163825e5749b303858fb902ebf198

Best regards,

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Hi Kees,

On Sat, 3 Dec 2022 at 03:45, Kees Cook <keescook@chromium.org> wrote:
> msc313_rtc_probe() was passing clk_disable_unprepare() directly, which
> did not have matching prototypes for devm_add_action_or_reset()'s
> callback argument. Refactor to use devm_clk_get_enabled() instead.

I didn't know devm_clk_get_enabled() existed. Learned something new. :)
Really nice that it fixes an issue and gets rid of a bunch of lines.

Anyhow, the change looks good to me, I build, boot and "does RTC still
work?" tested this so:

Reviewed-by: Daniel Palmer <daniel@thingy.jp>
Tested-by: Daniel Palmer <daniel@thingy.jp>

Cheers,

Daniel